effective diffusivity - springer · miller gq, stöcker j (1989) ... ies, such as ions dissolved in...

E

Ease of Expansion in Gas Separation

Adele BrunettiInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Quite often during the operation of a system forthe treatment of gases, it is necessary to expand itfor treating greater streams. In some cases, futureexpansion is contemplated even during the initialphase of a project. In other cases, it could be anecessity not foreseen during system design phase(Miller and Stöcker 1989; Brunetti et al. 2010).

Membrane system expansion is very easy,since this only requires the addition of identicalmodules. This is the advantage offered by themodularity of membrane units and the reducedequipment and control systems required for oper-ating it. In comparison, considering the other ref-erence technologies for gas separation, PSA andabsorption systems can also be expanded, but itrequires additional design considerations andadds cost in the initial phase of the project.

The cryogenic units cannot be expanded if it isnot foreseen during the design phase. Generallythey can be over-dimensioned, and a capacityincrease is often obtained without modificationto the cold box itself through addition of a tailgas compressor.

# Springer-Verlag Berlin Heidelberg 2016E. Drioli, L. Giorno (eds.), Encyclopedia of Membranes,DOI 10.1007/978-3-662-44324-8

References

Brunetti A, Bernardo P, Drioli E, Barbieri G (2010) Mem-brane engineering progresses and potentialities in gasseparations. In: Yampolskii Y, Freeman B (eds) Mem-brane gas separation. Wiley, New York, pp 281–312

Miller GQ, Stöcker J (1989) Selection of a hydrogen sep-aration process NPRA annual meeting, 19–21 Mar,San Francisco

Effective Diffusivity

Renzo Di FeliceUniversity of Genova, Genova, Italy

Effective diffusivity is a convenient parameterwhich is introduced when diffusion takes placein non-homogenous media. Consider, for exam-ple, the case where a species is diffusing throughporous particles, such as reactant diffusing insidea catalytic solid. In this case, the molecules haveto travel for a longer distance given that the poresof the catalytic particles are not straight, andmoreover diffusion takes place over a smallerarea due to the solid being wall impermeable.These effects are taken into account by definingan effective diffusivity as

Deff ¼ Det

� �

624 Effectiveness Factor

where e is the particle void fraction and therebytakes into account the reduced flow area availableand t is the tortuosity which considers the longerdistance traveled by the molecules. It should bestressed, however, that more often than not, theparameter inside the parenthesis is utilized as afitting factor derived from experimental datarather than predicting factor from the solid phys-ical characteristics. These result in the reportedtortuosity factor, for example, to assume ratherwild values, as high as ten, which are difficult tojustify with geometrical arguments alone.

The concept of effective diffusivity is alsoapplied for the case of species diffusing in hetero-geneous media, such a polymer filled with a sec-ond, less permeable, material. Maxwell (1873)has obtained an exact expression, in the case ofcomposite media filled with spheres, for the effec-tive diffusivity given as

Deff ¼ D

2

DSþ 1

D� 2f

1

DS� 1

D

� �2

DSþ 1

Dþ f

1

DS� 1

D

� �

where D is the diffusion coefficient in the primarymedia, Ds the diffusion coefficient through thespheres, and f the sphere volume fraction. Forthe limiting case of the spheres being completelyimpermeable, Maxwell expression simplify to

Deff ¼ D1� f

1þ f2

Various empirical expressions have been reportedin literature in order to take into account the devi-ation from sphericity of the foreign material. Par-ticularly simple and efficient is the expressionproposed by Nielsen (1967):

Deff ¼ D1� f

1þ af2

where a is the dispersed material aspect ratio.Nielsen relationship has been shown to do a sat-isfactory job against experimental evidence(Di Felice et al. 2008).

References

Di Felice R, Cazzola D, Cobror S, Oriani L (2008) Oxygenpermeation in PET bottles with passive and activewalls. Packag Technol Sci 21:405–415

Maxwell JC (1873) A treatise on electricity and magne-tism, vol 1. Clarendon Press, Oxford

Nielsen LE (1967) Models for the permeability of filledpolymer systems. J Macromol Sci A1(5):929–942

Effectiveness Factor

Renzo Di FeliceUniversity of Genova, Genova, Italy

The concept of effectiveness factor is utilizedwhen the effect of diffusion on the overall rate ofa reacting process wants to be taken into account.Let’s consider a catalytic solid particle where areaction takes place. For a species to react, it must,at the same time, diffuse inside the catalytic parti-cle and react. It is intuitive that the diffusion stepslows the overall process rate, and an effective-ness factor is defined as

�¼ actual rate of process

rate of process if diffusion were infinitively fast

This definition is equivalent to consider reactantconcentration on any point in the catalytic particleequal to that at the external surface.

A rather straightforward mass balance allowsthe determination of the effectiveness factor for asingle cylindrical pore where a first-order reactionis taking place on the wall:

� ¼ tanhff

where f is the Thiele module defined as

f ¼ L

ffiffiffiffiffiffiffiffik

Deff

s

with L the pore length, k the chemical reactionkinetic factor, and Deff the reacting species effec-tive diffusivity.

Electrical Double Layer 625

E

As expected the effectiveness factor approa-ches 1 when Thieles module is very small, i.e.,for very short pores or very slow reaction or veryhigh diffusion coefficients. On the other hand, forvery high value of Thiele modules, effectivenessfactor approaches the inverse of the Thielemodule.

The above result can be easily generalized forthe case of flat plate by putting L equal to half theplate width, for the case of long cylindrical pelletsby putting L equal to half the pellet radius, and tothe case of spherical pellets by putting L equal toone third the sphere radius. Extension to kineticdifferent from the first order has been presented inliterature, and a summary can be found, for exam-ple, in Satterfield (1991).

Electrical Double Layer, Fig. 1 Schematic sketch ofstructure of the Electrical Double Layer for (a)functionalized and (b) nonfunctionaalized membranematerials

References

Satterfield CN (1991) Heterogeneous catalysis in industrialpractice. McGraw-Hill, New York

Electrical Double Layer

Karel Bouzek and Tomas BystronFaculty of Chemical Technology, University ofChemistry and Technology Prague, Technická 5,Prague 6, Czech Republic

An electrical double layer is usually considered toform at the phase interface between two electri-cally conducting media. In such a case, on oneside of the interface, an excess of positive chargeexists, which is counterbalanced by an identicalexcess of negative charge localized on the oppo-site side of the interface. The overall systemcharge is thus equal to zero. In membrane science,this definition has, to a certain degree, been mod-ified, the reason being that membranes can beproduced from a broad spectrum of materials,including (i) electronic conductors (e.g., metals),(ii) ionic conductors (e.g., O2�-conductingceramics), (iii) functionalized polymers providingionic conductivity through the liquid filling the

void volume of the material (ion-selective mem-branes), and (iv) nonfunctionalized polymeric orceramic membranes.

Whereas, in the first two cases, the traditionaldefinition is valid, in the last two, the situation isdifferent. This is due to the fact that the bulkmembrane material is neither an electronic noran ionic conductor. Hence the double layer onlyforms on one side of the phase interface. TheFig. 1 provides a schematic sketch of the twomentioned cases. In the case of functionalizedmaterials, the charge-carrying groups covalentlybound to the polymeric backbone are orientedtowards the void volume of the membrane inte-rior, thus forming an electrically charged filmcovering the phase interface on the side of thesolution. The charge of this film is compensatedby that of the mobile ions present in the solutionand located close to the film (see Fig. 1a).

In the case of the non-functionalized materials,the surface charge fixed to the surface of the solidphase is formed by specific adsorption of ions ofone sign. Both the sign of the adsorbed ions andthe extent of the adsorption are determined by theproperties of the membrane material. In this case,too, the fixed charge is compensated by the charge

626 Electrical Interactions in Membranes

of ions moving in the system and located close tothe absorbed surface film (see Fig. 1b).

An electrical double layer significantly influ-ences the properties of the membrane, mainly withrespect to its transport properties and behaviorunder current load. In selected cases the surfaceof the membrane materials is modified to achievethe preferred adsorption and thus the requiredtransport and properties, including selectivity.

Electrical Interactions in Membranes

Karel BouzekFaculty of Chemical Technology, University ofChemistry and Technology Prague, Technická 5,Prague 6, Czech Republic

In general electrical (electrostatic) interactionsarise as a result of the interaction of electricallycharged particles (charge carriers) with sur-rounding matter (electrically charged and neu-tral) (Stuart 1989). In the case of membranes,probably the most important interactions to con-sider are those between electrically charged bod-ies, such as ions dissolved in solution andion-conducting, functionalized, or even inertmembranes. In living systems the electrostaticinteractions between the charged/polar and neu-tral/nonpolar side of a solute molecules with eachother and with a solvent are responsible for theformation/assembly of monolayer and bilayerbiological membranes and their functioning.

On the fluid side the charge carriers are typi-cally mobile ions or colloidal particles of organicand inorganic nature. These carriers interact withthe charge carriers fixed in the material of themembrane (see also the electrical double layer).The charge fixed in the membrane material is, inprinciple, of dual origin: (i) it comes from thecomponent introduced and incorporated duringits synthesis, and (ii) it is the result of the specificadsorption of ions on the surface of the solidphase. Electrostatic interactions between fixedand freely moving charge carriers are responsiblenot only for several phenomena characteristic of

membrane separation processes, such as the exis-tence of Donnan potential, the selectivity of masstransport, or mass transport enhancement undercurrent load by electroosmotic flux, but also forthe existence of life itself. These facts documentthe importance of electrical interactions in mem-brane materials with respect to their transportproperties.

References

Stuart McLaughlin (1989) The Electrostatic Properties ofMembranes. Annu. Rev. Biophys. Biophys. Chem.18:113–136

Electrical Potential

Karel Bouzek and Tomas BystronFaculty of Chemical Technology, University ofChemistry and Technology, Prague, Technická 5,Prague 6, Czech Republic

Electric potential is a scalar physical quantitydetermined by the electric energy (or work ingeneral) necessary to transfer a unit positive elec-tric charge from infinity to the point of interest in astudied system. The point in infinity is formallychosen as a reference state characterized by zeroelectric potential. From this more or less abstractand intuitive definition of the reference state, it isapparent that the absolute value of electric poten-tial is not available experimentally. However, it ispossible to measure electric potential difference,known as voltage.

We can distinguish between outer (external)and inner (internal) electric potential. In the caseof outer electric potential, the charge is transferredto the closest proximity of the external surface ofthe phase P. However, the distance from this sur-face has to be large enough to avoid the appear-ance of molecular and image forces. At the sametime it should not cause any weakening of theelectric interaction with the charge located on theinternal side of the phase interface. The inner

Electrically Enhanced Processes by Membrane Operations 627

electric potential is determined by the strength ofthe electric field in the interior of the phase P. Itthus represents the work necessary to transport thecharge from infinity to the phase P. The differencebetween the inner electric potential and the outerelectric potential corresponds to the surface poten-tial of phase P (Arthur and Alice 1997).

E
References
Arthur W. Adamson, Alice P. Gast (1997) ElecricalAspects of Surface Chemistry, in Physical Chemistryof Surfaces, 6th edn. John Wiley & Sons, Inc.ISBN:0-471-14873-3)

Electrical Work in Membranes


An electrical work is defined as the workconnected with the charge carrier transportbetween two points in an electric field, i.e., frompoint of potential ’1 to point of potential ’2. Thus,electrical work in membranes is the product of thetransported electrical charge and the difference inelectric potentials between which it has passed(Arthur and Alice 1997). Since the resistivity ofthe membrane can be approximated by ohmicresistivity, the electrical work dissipated in themembrane is equal to the ohmic potential loss inthe membrane multiplied by the current load.Expressed alternatively, it is equal to the squareof the current load multiplied by the ohmicresistivity.

References

Arthur W. Adamson, Alice P. Gast (1997) ElecricalAspects of Surface Chemistry, in Physical Chemistryof Surfaces, 6th edn. John Wiley & Sons, Inc. ISBN:0-471-14873-3

Electrically Enhanced Processes byMembrane Operations


Electrically membrane operations are typicallylimited to systems involving electrically chargedparticles. In such a case, the application of anelectric field can often significantly enhance theongoing process. In selected cases, it is the appli-cation of an external electric field that makes thedesired processes possible at all. By applying anelectric field, electrically charged particles aredragged in the direction determined by the polar-ity of their charge and the vector of the electricfield. This movement is responsible for electriccurrent flow through the system. Part of theelectric charge carried by one type of particlescorresponds to what is known as the transferencenumber of the given particle. It is proportional tothe particle mobility in the system and the electriccharge it carries (Colin 1976).

In the case of membrane operations enhancedby an electric field, generally functionalizedion-selective membranes are considered. Electro-dialysis and dialysis are typical representatives ofan electrically enhanced and a nonenhanced pro-cess, respectively. Electrodialysis is used to desa-linate a treated stream containing dissolved ionsby transporting them, due to the action of theelectric field, against the concentration gradientto the concentrate stream (Fernando et al. 2011).In the case of dialysis the situation is the reverse,since the dissolved species move in the directionof the concentration gradient away from thetreated stream to the solution free of the removedcomponents flowing along the opposite side of themembrane. It is thus clear that electrodialysis istypically a significantly more intensive process interms of the desalination rate. At the same time, itproduces a significantly lower volume of thestream carrying the removed components. Theelectric field thus enhances the intensity of the

628 Electrochemical Deposition

mass flux during membrane processes involvingcharged species and makes it possible to transportthe charged species against the concentration gra-dient. On the other hand, dialysis is not limited tocharged particles.

Electrically enhanced membrane operationscan also be carried out with nonfunctionalizedmembrane materials or with membrane materialscarrying no fixed electric charge, electrofiltrationbeing an example. Here, the applied electric fieldinfluences the membrane operation indirectly bypreventing blockage of the membrane surface.This is ensured by the electrophoretic transportof the charged colloidal particles concentrated inthe solution away from the surface of the filteringmembrane, see also Electrofiltration.

It is, therefore, evident that the application ofan external electrical field can significantlyenhance or modify processes, including mem-brane operations.

References

Colin A (1976) Vincent: The Motion of Ions in Solutionunder the Influence of an Electric Field. J. Chem. Educ,vol. 53, pg. 490

Fernando Valero, Angel Barceló and Ramońn Arboás(2011). Electrodialysis Technology - Theory andApplications, Desalination, Trends and Technologies,Michael Schorr (Ed.), ISBN: 978-953-307-311-8,InTech, DOI: 10.5772/14297. Available from: http://www.intechopen.com/books/desalination-trends-and-technologies/electrodialysis-technology-theory-and-applications

Electrochemical Deposition

▶Track-Etch Membranes as Tools for TemplateSynthesis of Nano-/Microstructures and Devices

Electrochemical ImpedanceSpectroscopy

▶ Impedance Spectroscopy, Membrane Charac-terization by

Electrochemical ImpedanceSpectroscopy (EIS)

▶ Impedance Spectroscopy

Electrochemical MembraneBioreactor

▶Bioelectrochemical MBR

Electrochemical Processing

Tanja Vidakovic-KochMax Planck Institute for Dynamics of ComplexTechnical Systems, Magdeburg, Germany

In electrochemical processing, electrical energy issupplied to or obtained from the electrochemicalsystem in order for chemical production or energyconversion to take place (Bard and Stratmann2007). The first group of processes, also calledelectrolytic, is not spontaneous. The second groupof processes, called galvanic, is spontaneous and itdelivers electrical energy. Electrolytic processescan be further divided into two categories: inor-ganic and organic. In inorganic electrochemicalprocessing, important commodity chemicals suchas sodium hydroxide, chlorine, and pure metals areproduced. The major inorganic electrochemicalprocessing technologies are chlor-alkali electroly-sis and electrowinning of metals like aluminum orcopper. Nowadays, hydrogen production by waterelectrolysis gets more on importance in context ofchemical storage of renewable electrical energy(wind and photovoltaic) in hydrogen. The mostsignificant commercial electroorganic synthesis isMonsanto’s electrohydrodimerization (EHD) ofacrylonitrile to adiponitrile. Adiponitrile has animportance in production of nylon 6-6. Examplesof galvanic systems are fuel cells and batteries. Themain “product” of galvanic systems is electricalenergy.

http://www.intechopen.com/books/desalination-trends-and-technologies/electrodialysis-technology-theory-and-applications




http://dx.doi.org/10.1007/978-3-662-44324-8_473

http://dx.doi.org/10.1007/978-3-662-44324-8_473

http://dx.doi.org/10.1007/978-3-662-44324-8_863

http://dx.doi.org/10.1007/978-3-662-44324-8_863

http://dx.doi.org/10.1007/978-3-662-44324-8_1886

http://dx.doi.org/10.1007/978-3-662-44324-8_2156

Electrochemical Regeneration 629

E

Electrochemical systems have some intrinsicadvantages over other types of chemical systemslike better control of a reaction rate, operation atlower temperatures, and less environmentalimpact. They take place in an electrochemicalreactor. The design of an electrochemical reactoris influenced by the state of aggregation of reac-tants (gas, liquid, or solid), necessity of reactantsand/or products separation, required mass trans-port conditions, and electrode materials. If prod-uct or reactant separation is required, anelectrochemical reactor must contain a separator,which is a membrane. Major requirements on amembrane are good separation efficiency, lowelectrical resistance, no electron conductivity,low cost, long operating life time, good dimen-sional stability, and resistance to plugging andfouling. In general permeable and semipermeablemembranes have been applied in electrochemicalprocessing. Permeable membranes are porousmaterials filled with liquid electrolyte which per-mit the bulk flow of liquid through their structureand are thus nonselective regarding transport ofions or neutral molecules. In electrochemical pro-cesses, these are also referred to as diaphragms.Permeable membranes can be made of inorganicand organic materials and composites. Examplesof these materials are asbestos (chlor-alkali elec-trolysis), polymers like polyethylene and polypro-pylene (batteries), or composites like polymer(polypropylene)-modified asbestos. Semiperme-able membranes permit the selective passageof certain species by virtue of molecular sizeor charge. In electrochemical processes,ion-conducting membranes (see solid electrolyte)are broadly applied. In general, ion-conductingmembranes have higher separation efficiencyand lower electrical resistance than diaphragms,but they are also more costly and impose higherrequirements on system purity.

References

Bard AJ, Stratmann M (eds) (2007) Encyclopedia of elec-trochemistry. Macdonald DD, Schmuki P (eds) Elec-trochemical engineering, vol 5. Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim

Electrochemical Regeneration


In electrochemical regeneration electrical energyis applied to restore some important property, likeadsorption capacity or catalyst activity of a tech-nical system. Electrochemical regeneration relieson principles of electrochemistry and relates toelectrochemical processing. Electrochemicalregeneration is conveniently conducted in situwith an electron as only reagent requiring simplehandling and equipment. A technical setup forelectrochemical regeneration requires in generaltwo electrodes, an electrolyte and a power supply.In addition a membrane can be added to the setupin order to separate anode and cathode depart-ment. Some examples of electrochemical regen-eration are electrochemical regeneration ofactivated carbon-based adsorbents in wastewatertreatment and regeneration of enzymatic cofactorsin electroenzymatic processes.

Organic pollutants in wastewaters can beremoved by adsorption using, e.g., activated car-bon as an adsorbent. This process is normallyoperated using a batch of adsorbent with sufficientcapacity to operate for many months beforereaching saturation. Once loaded, adsorbentmust be disposed or regenerated. One option foradsorbent regeneration is electrochemical regen-eration (Brown et al. 2004). The loaded adsorbentis located in a form of a packed or fluidized bed inthe anode (anodic regeneration) or cathode(cathodic regeneration) compartment of the reac-tor. The efficiency of the regeneration depends onthe processing time, voltage gradient, an electro-lyte, and a compartment. According to literaturethe efficiency of cathodic regeneration is higherthan of anodic regeneration. The mechanism ofelectrochemical regeneration is ascribed at thefirst place to local pH changes close to anode orcathode. At the anode side due to oxygen evolu-tion reaction a pH decrease can be expected, whileat the cathode side due to hydrogen evolution pH

630 Electrochemistry

value will increase. This pH changes induceorganic pollutants desorption. In the next step,dissolved pollutants can be oxidized at the anode.In the case of the cathodic regeneration they havefirst to mitigate from the cathode to the anode. Thismight be mass transfer controlled leaving someresidues in the cathode, unless very large currentsor long regeneration times are employed.

Further example of electrochemical regenera-tion is regeneration of enzymatic cofactors inelectroenzymatic processes (Wichmann andVasic-Racki 2005). Redox enzymes are veryselective and specific catalysts, which can enablea number of partial oxidation or reduction reac-tions for industrial applications at mild conditions.Broader industrial application of redox enzymeshas been so far hindered by their dependence onexpensive cofactors (e.g., nicotinamide adeninedinucleotide (NAD)), which are consumed in thereaction (e.g., Eq. 1) and have to be regeneratedfor a process to be economical:

CO2 þ NADH þ Hþ ⇄ HCOOH þ NADþ (1)

Electrochemical regeneration offers a possibilityof cofactor regeneration. In this respect especiallyregeneration of NAD has been studied sinceNAD-dependent oxidoreductases are of greatindustrial interest. The electrochemical regenera-tion can be represented by this reaction:

NADþ þ Hþ þ 2e� ⇄ NADH (2)

This reaction is however not selective enough andthe kinetics is very sluggish on most known elec-trode materials. Some improvements have beenachieved by using surface-modified electrodes.Another strategy is to add an additional mediatoraccording to

NADþ þ Medred ⇄ NADH þ Medoxþ (3)

Medoxþ þ Hþ þ 2e� ⇄ Medred (4)

Electrochemical cofactor regeneration is still not amature technology, and further improvements inelectrode materials are needed to make this optionfeasible.

References

Brown NW, Roberts EPL, Garforth AA, Dryfe RAW(2004) Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye.Electrochim Acta 49:3269–3281

Wichmann R, Vasic-Racki D (2005) Cofactor regenerationat the lab scale. Adv Biochem Eng Biotechnol92:225–260

Electrochemistry


Electrochemistry is a branch of chemistry whichstudies charge transfer processes across an elec-trified interface also called an electrochemicaldouble layer (Bockris and Reddy 1988, Hamannet al. 2007). Applications of electrochemistry arebroad including electrochemical processing, elec-troanalysis electrochemical sensors, electrochem-ical regeneration, and corrosion. In addition,many important processes in biological systemslike photosynthesis and cell respiration are inher-ently electrochemical processes.

The main feature of an electrochemical systemis a separation of ionic- and electronic flows. Ionsare flowing through the electrolyte which is exclu-sively an ionic conductor, while electrons flowthrough an outer electrical circuit which is exclu-sively an electron conductor. These two flows areinterconverted at the electrode/electrolyte inter-face across the electrochemical double layer bymeans of an electrochemical reaction The poten-tial difference in the electrochemical double layeris related to thermodynamics (Nernst equation)and kinetics (Butler-Volmer or Tafel equations)of an electrochemical reaction and it is a drivingforce for the electrochemical reaction to takeplace. This unique feature of electrochemistrymakes easy to control the reaction rate by elec-trons at different energies.

Electrochemical processes can be spontaneous(Gibbs free energy, DG <0), called galvanic, and

Electrodeionization 631

E

nonspontaneous DG>0, called electro-lytic. Instead in terms of Gibbs free energy, spon-taneity of an electrochemical process can beexpressed in terms of cell voltage, where a posi-tive value stands for a galvanic system and anegative for an electrolytic. The relationshipbetween the cell voltage and Gibbs energy isgiven by equation DG = �nFUr, where n standsfor number of exchanged electrons, F for a Fara-day constant, and Ur for an equilibrium cellvoltage.

Many electrochemical systems require pres-ence of separators. This is usually a membranewhich can be a permeable, termed diaphragm, orsemipermeable, termed membrane. The latter typeusually in addition to separation serves as anelectrolyte, so-called solid electrolyte in electro-chemical systems. An example is ceramic yttria-stabilized zirconia (YSZ) membrane which hasfound an application in solid oxide fuel cells.The ionic conductivity of this material is providedby O2� ions.

References

Bockris JO’M, Reddy AKN (1988) Modern electrochem-istry. Plenum Press, New York

Hamann CH, Hamnett A, VielstichW (2007) Electrochem-istry, 2nd edn. Wiley-VCH Verlag GmBH, Weinheim

Electrodeionization


Electrodeionization represents a variant of elec-trodialysis, modified in order to allow treatment oflow-salinity and low-conductivity media. Thistechnique is typically applied to produce high-purity water suitable for use, for example, in ener-getics. It combines the advantages of ionexchange with those of electrodialysis.

This technology is based on an electrodialysisunit with a diluate and also, in selected cases, aconcentrate chamber filled with ion-exchangerparticles. They can be arranged as monopolarbeds (formed by particles of one polarity ionexchanger), as layered beds (cation- and anion-exchanger particles filled separately in severalalternating layers), or as a mixed bed (uniformmixture of both types of ion-changer particles).The ion-exchange phase takes on the role ofelectroconductive media, thus reducing ohmicdrop in the dilute chamber. At the same time itprovides a three-dimensional interface for theremoval of traces of ions present in the solution.Two regions are typically distinguished in theelectrodeionization operation: (i) ions removaland (ii) solvent splitting. Within the first regionthe electrodeionization unit works bellow masstransfer limitation. It means, flux of ions to thesolution – ion exchanger interface driven by thecurrent load used has a value well below masstransfer limitation in a dilute chamber. The func-tion of the ion-exchange bed consists in providinga pathway for ions trapped in the dilute channel tothe ion-selective membranes separating dilute/concentrate chambers. In a second domain, how-ever, the current load exceeds limiting currentdensity, i.e. limiting flux of ions present fromsolution to the solution – ion exchanger interface.In such a case sufficient number of ions to trans-port corresponding electrical charge is providedby decomposition (dissociation) of the solvent(typically water). In contrast to electrodialysis,this splitting does not take place only at thesolution-membrane interface but also at the con-tact of the cation- and anion-selective phase(Alvarado and Chen 2014).

In the case of a concentrate chamber, the role ofthe ion-exchange phase again consists in reducingohmic drop in the channel while maintaining theconcentration of the ions in the liquid phase at aminimum to reduce back diffusion from the con-centrate to the dilute chamber.

The quality of the stream produced is compa-rable to that of the ion-exchange process. Theadvantage of electrodeionization is that it is acontinuous process that does not require a regularregeneration phase of operation. This feature of

632 Electrodialysis

electrodeionization has a further importantadvantage. It saves a significant amount ofcorresponding chemicals and reduces the salinityof the waste streams produced. Such technology isthus a suitable component for closed loop tech-nologies which, on ending, discharge removedsalts in the solid form and avoids production ofcontaminated liquid streams.

References

Alvarado L, Chen A, (2014) Electrodeionization: Princi-ples, Strategies and Applications, Electrochim. Acta132:583

Electrodialysis

Heiner StrathmannUniversität Stuttgart, Institute of ChemicalProcess Engineering Stuttgart, Baden-W€urttemberg, Germany

The principle of electrodialysis is illustrated inFig. 1 which shows a series of alternating anion-and cation-exchange membranes arrangedbetween two electrodes. The membranes are sep-arated by a spacer gasket and form individualcells, through which an electrolyte solution ispumped. When an electrical potential difference

Feed

Electroderinse

+

Anode

++

+

–

+ –

++++++++++

Concentrate

Diluate

A

Electrodialysis,Fig. 1 Schematic diagramillustrating the principle ofelectrodialysis

between the electrodes is established, the cationsmigrate toward the cathode. They pass throughthe cation-exchange membrane, but they areretained by the anion-exchange membrane. Like-wise the anions migrate toward the anode and passthrough the anion-exchange membrane but areretained by the cation-exchange membrane. Theoverall result is that an electrolyte is concentratedin alternate compartments, while its ion content isdepleted in the other compartments. In an indus-trial size electrodialysis stack, 100–400 cell pairsare arranged between the electrodes. Variousspacer and stack constructions such as theso-called sheet flow are used in practical applica-tions (Schaffer and Mintz 1966).

The concept of a sheet flow stack is illustratedin Fig. 2 which shows the arrangement of thespacers and membranes in a stack. The spacersnot only separate the membranes and provide theproper mixing of the solutions in the cells, but intheir frames, they also contain the manifolds forthe two different flow streams in the stack.

Practical problems which affect the efficiencyof electrodialysis are concentration polarizationand membrane fouling. Concentration polariza-tion which determines the so-called limiting cur-rent density is controlled by hydrodynamicparameters of the feed flow in the stack. Mem-brane fouling is controlled by a technique which isreferred to as “clean in place” or electrodialysisreversal (Katz 1979).

The total costs in electrodialysis are the sum ofthe plant capital and the plant operating costs. The

Repeating unit

C A C C

Electroderinse

–

Cathode

++

+

–

+ –

++++++++++

––

–

–+

–––––––––

+ ––

–

–+

–––––––––

+ ––

–

–+

–––––––––

+

Electrodecell

Ion-exchangemembrane

Concentrate

Diluate

Feed solution

Feed solution

Electrode Diluate cellSpacer Concentrate cell

Electroderinse solution

Electrodialysis, Fig. 2 Schematic drawing illustrating the construction of a sheet flow stack design

Electrodialysis Reversal 633

E

capital related costs are proportional to therequired membrane area for a given capacityplant and a for a given feed and productconcentration. The operating costs are propor-tional to the required desalination energy whichis the function of the transferred ions, i.e., theconcentration difference between the feed andproduct ion concentration. The energy EV

required to desalinate 1 m3 of water by electrodi-alysis is given by

EV ¼ FrACP i

xCf � Cp� �

Here rCPA is the area resistance of the cell pair,

which can be estimated from measurements ofmembrane resistance and conductivities of solu-tions, F is the Faraday constant, i is the currentdensity, Cf and Cp are the feed and product waterconcentrations, and x is an efficient term which isgenerally close to 1.

Electrodialysis has advantages and limitationscompared to other deionization processes such asreverse osmosis. A main advantage of electrodi-alysis compared to reverse osmosis is that verylittle feed pretreatment is required and higherbrine concentrations can be achieved.

Amajor disadvantage especially for the produc-tion of potable water is the fact that only ions areremoved, while uncharged components such asmicroorganisms or organic contaminants will notbe eliminated. Another disadvantage of electrodi-alysis is the relatively high energy consumptionwhen solutions with high salt concentrations haveto be processed. Thus, electrodialysis can only becost-effectively applied in water desalination in acertain range of feed water salt concentration andrequired product water (Strathmann 2010).

References

Katz WE (1979) The electrodialysis reversal process.Desalination 28:31–40

Schaffer LH, Mintz MS (1966) Electrodialysis. In:Spiegler KS (ed) Principles of desalination. Academic,New York, pp 3–20

Strathmann H (2010) Electrodialysis, a mature technologywith a multitude of new applications. Desalination264:268–288

Electrodialysis Reversal

▶Reverse Electrodialysis (RED)

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634 Electrodialysis Reversal in High Supersaturation Mode

Electrodialysis Reversal in HighSupersaturation Mode

▶Reverse Electrodialysis in High Supersatura-tion Mode

Electrodialysis with BipolarMembranes

Heiner StrathmannUniversität Stuttgart, Institute of ChemicalProcess Engineering Stuttgart, Baden-W€urttemberg, Germany

The conventional electrodialysis can be combinedwith bipolar membranes and utilized to produceacids and bases from the corresponding salts.A bipolar membrane is a laminate of an anion ona cation-exchange layer. In this processmonopolar cation- and anion-exchange mem-branes are installed together with bipolar mem-branes in alternating series in an electrodialysis

Base Acid

am

Repeatincell uni

X−X−+

bpm

Salt solution Salt soluti

M +M +H +

cm

OH−

––––––––––––––––

––––––––––––––––

++++++++++++++++

++++++++++++++++

Electrodialysis with Bipolar Membranes,Fig. 1 Schematic drawing illustrating the principle ofthe electrodialytic production of an acid and a base fromthe corresponding salt with bipolar membranes. Repeating

stack as illustrated in Fig. 1 which shows a typicalrepeating unit of an electrodialysis stack withbipolar membranes is composed of three cells,two monopolar membranes and a bipolar mem-brane. The outer cells of the repeating unit are fedwith a salt solution, the inner cells with water, or adiluted acid and base. When an electrical potentialgradient is applied across a repeating unit, protonsand hydroxide ions which are generated in thebipolar membrane generate with the cations andanions removed from the salt solution, an acid anda base on either side of the bipolar membrane. Theprocess design is closely related to that of theconventional electrodialysis using the sheet flowstack concept (Liu et al. 1977; Simons 1993).

The utilization of electrodialysis with bipolarmembranes to produce acids and bases from thecorresponding salts is economically very attractiveand has a multitude of interesting potential appli-cations in the chemical industry as well as in bio-technology and water treatment processes. Its keycomponent is the bipolar membrane. The bipolarmembrane schematically illustrated in Fig. 2 con-sists of a laminate of an anion- and a cation-exchange membrane with a 4–5 nm thick catalytictransition layer in between. In Fig. 2 this transition

AcidBase

am

–

gt

cm

OH−

X−

H+

on Salt solution

M +

bpm––––––––––––––––

––––––––––––––––

++++++++++++++++

++++++++++++++++

cell unit consisting of a cation-exchange membrane (cm), abipolar membrane (bpm), and an anion-exchange mem-brane (am)

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Cathode

Bipolar membrane

H2O H2O

H+OH–

Cation-exchangelayer

Anion-exchangelayer

Anode

–––––––– +

+++++++

Electrodialysis with Bipolar Membranes,Fig. 2 Schematic drawing illustrating the electrodialyticwater dissociation in a bipolar membrane with water dif-fusing into the reaction region between the cation- andanion-exchange layers of the membrane and protons andhydroxide migrate to the corresponding electrode

Electrodialysis with Bipolar Membranes 635

E

layer has been artificially magnified. Water is dif-fusing through both membrane layers into the tran-sition layer where it gets electro-catalyticallydissociated into H+- and OH�-ions, whichmigrate toward cathode and anode into the outersolutions.

The energy required for the water dissociationcan be calculated from the Nernst equation for aconcentration chain between solutions of differentpH-values. It is given by:

DG ¼ FD’ ¼ 2:3RTDpH

Here DG is the Gibbs free energy and DpH andD’ are the pH-value and the potential differencebetween the two solutions separated by the bipolarmembrane. For 1 mol/L acid and base in the twophases separated by the bipolar membrane, DG is0,022 kWh/mol and D’ is ca. 0,83 V at 25 �C.Compared to the ohmic potential drop over themembranes, the required potential drop for watersplitting in the transition layer is much more pro-nounced. The determination of the costs for theproduction of acids and bases from thecorresponding salts follows the same general pro-cedure as applied for the cost calculation in elec-trodialysis desalination. The overall costs are theinvestment-related costs and the operating costs.The investment-related costs are dominated by themembrane costs and are proportional to the

required membrane area for a given capacityplant. They are a function of the current densityapplied in a given stack operation. A unit cellcontains a bipolar membrane, a cation- and ananion-exchange membrane. The bipolar mem-brane is rather expensive, and its useful life timeas well as that of the anion-exchange membrane israther limited in strong bases. The operating costsin electrodialysis with bipolar membranes arestrongly determined by the energy requirementswhich are composed of the energy required for thewater dissociation in the bipolar membrane andthe energy necessary to transfer the salt ions fromthe feed solution and protons and hydroxide ionsfrom the transition region of the bipolar mem-brane into the acid and base solutions. The energyconsumption due to the pumping of the solutionsthrough the stack can generally be neglected.Since bipolar membranes became available ascommercial products, a large number of applica-tions have been identified and studied on a labo-ratory or pilot plant scale. However, in spite of theobvious technical and economical advantages ofthe technology, large-scale industrial plants arestill quite rare (Gineste et al. 1996). The mainreasons for the reluctant use of bipolar membraneelectrodialysis are poor membrane stability atvery high or low pH-values and insufficientpermselectivity at high ion concentrations, whichresults in a substantial product salt contamination,low current efficiency, and short membrane life.Nevertheless, there are a number of smaller-scaleapplications in the chemical process industry, inbiotechnology, in food processing, and in waste-water treatment.

References

Gineste JL, Pourecelly G, Lorrain Y, Presin F, Gavach C(1996) Analysis of factors limiting the use of BPM: asimplified model to determine trends. J Membr Sci112:199–208

Liu KJ, Chlanda FP, Nagasubramanian KJ (1977) Use ofbipolar membranes for generation of acid and base: anengineering and economic analysis. J Membr Sci2:109–124

Simons R (1993) Preparation of high performance bipolarmembranes. J Membr Sci 78:13–23

636 Electrodriven Membrane Processes

Electrodriven Membrane Processes

Seung-Hyeon MoonSchool of Environmental Science andEngineering, Gwangju Institute of Science andTechnology (GIST), Gwangju, South Korea

Introduction

Electrolyte membranes or ion exchange mem-branes (IEMs) are ion-selective membranes forseparation or charge transfer through the mem-branes. Electrodriven membrane processesemploy IEMs under an electric field to perform(i) mass separation for concentration or dilution ofions in aqueous phase; (ii) chemical synthesiswith an electrochemical reaction, such aschlorine-alkaline electrolysis and production ofhydrogen via water electrolysis; and (iii) energyconversion and storage involving the conversionof chemicals into electrical energy and vice versa.Non-charged porous membranes are also used insome energy conversion processes to separateanolyte and catholyte solutions.

Separation Processes

Applications of the first type are mainly found inseparation of ionic species under an electric field.Cations move toward a cathode and permeatethrough a cation exchange membrane whileanions move toward an anode and permeate ananion exchange membrane. Due to rejection ofnon-permeable ion by the membranes, electro-lytes are concentrated or diluted in a compart-ment. Typical processes in this category areelectrodialysis, bipolar membrane electrodiaysis,electrodeionization, and membrane capacitivedeionization. Without an electric field, diffusiondialysis enables to purify acid or base solutionscontaining metallic contaminants.

Electrodialysis (ED) is an electrochemicalseparation process using an electrical potential asa driving force. Electrodialysis systems typicallyconsist of a cell arrangement with a series of

alternating anion and cation exchange membranesbetween an anode and a cathode. Within ionexchange membranes, charged groups areattached to the polymer backbone of the mem-brane material. These fixed charged groups par-tially or completely exclude ions of the samecharge (co-ions) from the membrane, so that ananion exchange membrane (AEM) with fixed pos-itively charged groups excludes positive ions butis freely permeable to negatively charged ions,referred to as counterions. Similarly, a cationexchange membrane (CEM) with fixed negativelycharged groups is freely permeable to positivelycharged ions. When an aqueous salt solution iscirculated in the cell under an electrical potential,the positively charged cations migrate toward thecathode and the negatively charged anions towardthe anode. The overall result is a potential dropacross the cell pairs as well as a change in the ionconcentration in alternate compartments. Thedepleted solution is generally referred to as thediluate and the concentrated solution as the con-centrate. Electrodialysis is used for desalination ofvarious salt solutions including seawater as wellas production of table salt (Fig. 1).

A bipolar membrane (BPM) in which theanion- and cation-exchangeable layers(AEL/CEL) are adjoined together easily splitswater molecules into protons and hydroxyl ionsunder a reverse bias condition. Accordingly intro-duction of inorganic substances into the bipolarmembrane is known to be an effective techniquefor generation of acid and base. Two or threecompartment stack configurations may be chosendepending on the product specification, i.e.,CEM-BPM, AEM-BPM, or CEM-BPM-AEM. Itis expected that water-splitting electrodialysisusing bipolar membrane can make zero-emissionpossible and be applied as a clean technology byrecycling salt by-products after splitting to acidand base. Developments of high-performancebipolar membranes further expand use of bipolarmembrane electrodialysis in chemical, biochemi-cal, and environmental industries (Fig. 2).

Electrodeionization (EDI) or continuouselectrodeionization (CEDI) is a hybrid separationprocess that removes ionized species from liquidsusing electrically active media and an electrical

Electrodriven MembraneProcesses, Fig. 1 AnElectrodialysis Cell

Electrodriven MembraneProcesses, Fig. 2 ABipolar MembraneElectrodialysis Cell

Electrodriven Membrane Processes 637

E

potential activating ion transport. The electricallyactive media, such as an ion exchange resin inCEDI devices, may function to alternately collectand discharge ionized species or to facilitate thetransport of ions continuously by ionic or elec-tronic substitution mechanisms. Unlike an ionexchange process, CEDI does not requirechemicals to regenerate the ion exchange resinor concentrate the wastewater. In a CEDI system,the ion exchange resin bed plays a major role inthe reduction of the high electrical resistance inthe diluate compartment, while the ion exchangemembranes lead to depletion and concentration ofthe solutions in the diluate and concentrate com-partments, respectively. The CEDI process iscapable of achieving high levels of purification

and the concentration of dissolved ionic solutes ata relatively low salt concentration (Fig. 3).

Membrane capacitive deionization (MCDI)is an advanced capacitive deionization (CDI) withthe help of ion exchange membranes. A CDI sys-tem is operated by adsorption and desorption ofions on carbon electrodes.When an electric poten-tial is applied to CDI cells, charged ions in con-taminant water are adsorbed onto the surface ofcharged electrodes and forms an electric doublelayer due to the charged electrode and adsorbedions, producing purified water. Often CDI leads toa higher energy consumption and a lower opera-tion efficiency due to mobility of unwanted ions.To avoid this phenomenon, ion exchange mem-branes are introduced to CDI for ion selectivity,

Electrodriven MembraneProcesses, Fig. 3 AnElectrodeionization Cell


which is called membrane CDI (MCDI). AMCDIemploys two types of ion exchange membranes,i.e., cation exchange and anion exchange mem-branes. The AEM and CEM are positioned infront of the positively and negatively chargedelectrodes, respectively. While counterions areattracted onto the electrode surface, simulta-neously co-ions expelled from the counter elec-trode. The selectivity of ion exchange membranesprevents reverse adsorption and prohibits themobility of unwanted ions.

Diffusion dialysis is a membrane separationprocess in which transport of selective ions isdriven by a concentration difference over an ionexchange membrane. The diffusion dialyzer foracid recovery consists of a multitude of compart-ments made of gaskets arranged alternately withanion exchange membranes, which allow acids topermeate but retain metal salts. The gasket sur-rounds a meshlike spacer by which mixing in a

compartment is enhanced. The acid feed enters thebottom of every alternate compartment, as wateris fed from the upper part of the stack to contactthe acid feed counter currently. Permeate acids arecollected from the bottom of the permeate com-partment as purified acid product. The depletedfeed flows out of the upper part as an effluentcontaining high concentrations of metal salts anda low concentration of remaining acid. Similarly acation exchange membrane is used to recover basefrom metal salts.

Chemical Synthesis Processes

Applications of the second types are electrolysisprocesses to generate chemicals by electrode reac-tions and ion substitution through membranes.

A chlor-alkali process involves electrodereactions for the electrolysis of NaCl solution.

Electrodriven Membrane Processes, Fig. 4 A Membrane Capacitive Deionization Cell


E

The process produces caustic soda (NaOH) andchlorine (Cl2) that are used in a wide range ofindustries. The most common chlor-alkali processis the electrolysis of aqueous sodium chloride in amembrane cell. The membrane is a cation perme-able one which allows positive ions to passthrough it. That is, the only the sodium ionsfrom the NaCl solution may pass through themembrane and not the chloride ions. The advan-tage of the membrane process is that the sodiumhydroxide solution is formed in a cathode com-partment which is not contaminated with NaClsolution. Also purified NaCl should be suppliedto obtain pure NaOH solution. Chlorine is gener-ated at anode by oxidation of chloride ion. As aclean energy, hydrogen had been drawing moreattention. One of hydrogen sources is electrolysisof water, splitting water to hydrogen and oxygen.Various catalysts and energy sources are investi-gated to improve the efficiency as a reverse pro-cess of a fuel cell.

Electro-ion substitution reaction can be car-ried out in an ED stack in which membranes are allof the same kind, either anion or cation exchangemembranes. In a unit cell consisting of two com-partments with CEMs, acid and feed streams flow.When an electric current is supplied to the stack,protons in the acid stream (HX) are transported tothe feed stream (NaA) and convert the negativelycharged organics (A�) to free acid (HA). As a resultthe inorganic cations in the feed stream, in this casesodium ions, are transported to the acid compart-ment to meet the electroneutrality (NaX) in the feedstream, completing an ion exchange reaction,

NaA + HX ! HA + NaX. A four-compartmentdouble decomposition cell consists of a series ofanion and cation exchange membranes arranged inan alternating pattern between anode and cathode.When the cell is operated with four differentstreams, ion substitution or salt metathesis reactionoccurs, AX + BY ! AY + BX (Fig. 4).

Energy Conversion Processes

Applications of the third types are found in vari-ous energy conversion and storage systems wherecharge carriers are selectively permeate through amembrane. Charge carriers are generated fromelectrode reactions or initially supplied assupporting electrolytes. When charge carriersmove between anolyte and catholyte withoutmixing of active materials, a separator type mem-brane may be used. This type of applicationsincludes fuel cell, lithium ion battery, redox flowbattery, reverse electrodialysis, etc.

Polymer electrolyte fuel cells (PEFCs) arecommonly considered to be one of the most likelypotential alternative power sources due to theirconvenient operating temperature, high-energyconversion efficiency, and ease of integrationinto hybrid systems. Electricity is generated bypotential difference between anode where hydro-gen or fuel is oxidized to proton with the help ofcatalyst and cathode where oxygen is reduced andreacts with proton. A polymer electrolyte mem-brane is placed between the anode and cathodecatalysts to insulate between electrodes. Polymer

Electrodriven MembraneProcesses, Fig. 5 ADiffusion Dialysis Cell


electrolyte membrane (PEM) sandwiched withtwo catalyst layers is called a membrane-electrodeassembly (MEA). Perfluorinated sulfonic acidmembranes such as Nafion®, Flemion®,Aciplex®, and Dow® series have been widelyused due to their high proton conductivity andgood thermal and chemical stability.Hydrocarbon-based membranes, however, aregenerally less stable and subject to fast degrada-tion, yet to be improved for practical applications.Recent efforts have focused on the commerciali-zation of PEFCs in a range of applications, includ-ing use in portable devices, residences, stationaryapplications, and vehicles (Fig. 5).

A lithium ion battery is a rechargeable batteryin which lithium ions move between anode andcathode with redox reactions during charge anddischarge cycles. A separator is required to pre-vent mixing of anolyte and catholyte while lith-ium ions permeate freely in electrolyte solution.Often the separator is prepared as a multilayerporous membrane due to requirement of shut-down and meltdown. The lithium ion battery israpidly growing due to the widespread use ofwireless electronics such as cell phones and laptopcomputers. Moreover, concerns about globalwarming and oil shortages have led to the devel-opment of high energy density Li ion batteries

used for energy storages and electric vehicles. Inthese applications, safety is a key parameter. Thesafety of battery is closely linked to the batteryseparator absorbing a highly flammable liquidelectrolyte. Although typical separators based onsemicrystalline polyolefin materials such as poly-ethylene (PE) and polypropylene (PP) aremechanically strong and relatively inexpensive,their low melting temperature possibly causesphysical contact between two electrodes over180 �C. The physical contact triggers thermalrunaway, which results in explosive burning ofthe highly flammable liquid electrolyte (Fig. 6).

A redox flow battery is an energy storagedevice in which potential difference of a redoxcouple generates (discharge) or stores (charge)electric energy. Positive and negative electrolytesare stored separately and the electrolyte solutionsare fed into a cell for energy conversion. Avarietyof membranes are used for RFB systemsdepending on the charge carrier, electrolyte sol-vent, and redox couple. Membranes must possesstwo main properties: preventing the undesirablecross-mixing of the negative and positive electro-lytes to minimize self-discharge, while selectivelyconducting the charge carrier ions through themembrane to complete the redox circuit duringthe passage of current. Typically monovalent

Electrodriven MembraneProcesses, Fig. 6 Electro-Ion Substitution Cell

Electrodriven MembraneProcesses, Fig. 7 A SaltMethathesis Cell


E

selective cation exchange membrane or a thinanion exchange membrane is used to preventcrossover of the cationic redox couples when pro-ton is a charge carrier. A RFB is a promisingtechnology due to several advantages such as itslarge capacity, convenient operating temperature,long cycle life, and so on. On the other hand, theuse of an aqueous electrolyte severely restricts theenergy density as is determined by the cellpotential that is constrained by the electrochemi-cal stability window of water. Along with com-mercialization of aqueous RFBs, nonaqueousRFBs which can offer a higher density havedrawn considerable interest from energy storage

researchers. Furthermore, nonaqueous organicelectrolytes provide a wider potential window ofoperation due to the high solubility of variousmetal–ligand complexes in an organic solvent.Nonaqueous RFB systems require ion exchangemembranes or non-charged porous membranesdepending on the electrochemical propertiesand molecular size of charge carriers employed(Fig. 7).

Reverse electrodialysis (RED) is an ionexchange membrane process for renewableenergy generation. Electric potential occurs as aresult of Donnan equilibrium where two solutionscontaining permeable ions are separated by a

Electrodriven MembraneProcesses, Fig. 8 APolymer ElectrolyteMembrane Fuel Cell

642 Electroendosmosis

corresponding ion exchange membrane. Using asimilar cell structure to electrodialysis, electricitycan be generated by diffusion of ions using con-centrated salt solution and diluted solution, calledreverse electrodialysis (RED). Typically seawaterand river water may be used as feed solution in amodified electrodialysis cell. Other salinitygradient power generation is pressure-retardedosmosis using pressure difference due to theconcentration difference across an osmotic mem-brane including a polymer electrolyte active layer(Fig. 8).

Perspectives

Electrodialysis has been studied and industrial-ized for nearly a century as a leading electrodrivenmembrane process. Along with the appearance ofadvanced ion exchange membranes, various sep-aration and reaction processes are following elec-trodialysis in many industries as demands forcompact and environmentally friendly processesare growing continuously. Last several decadesenergy-related electrodriven membrane processesare growing rapidly with the help of membranesas a key component. Many membranes originallydeveloped for separation processes haveexpanded their applications for energy conversionand storage. Accordingly the membranes shouldbe characterized in terms of electrochemical prop-erties to improve the performance appropriately,

i.e., ionic conductivity, transport number, oxida-tive stability, and power density.

Electroendosmosis

▶Electroosmosis, Overview of

Electro-Fenton Process

Marc Cretin1,2 and Mehmet A. Oturan31Institut Européen des Membranes,ENSCM-UM-CNRS (UMR 5635), Université deMontpellier, Montpellier Cedex 5, France2National School of Chemistry of Montpellier,University of Montpellier, Montpellier, France3Laboratory of Earth Materials and Environment,University Paris-Est, Marne La vallée,Champs-sur-Marne, France

The electro-Fenton process is one of the mostpopular electrochemical advanced oxidation pro-cesses. As all the advanced oxidation process(AOPs), electro-Fenton (EF) process is based onthe in situ production and reactions of the hydroxylradicals (•OH), the second powerful oxidant (afterfluorine) with E0 (•OH/OH�) = 2.80 V/SHE and avery reactive species (Brillas et al. 2009; Oturan

http://dx.doi.org/10.1007/978-3-662-44324-8_2079

Electrofiltration 643

and Aaron 2014). •OH is able to mineralize anyorganic pollutants and therefore used in AOPs inorder for destruction of persistent organicpollutants.

Contrary to anodic oxidation process (Panizzaand Cerisola 2009) in which •OHs are produced atanode surface from water oxidation (Panizza andCerisola 2009), in the EF process, •OHs are gen-erated in the bulk of solution from electrochemi-cally assisted Fenton’s reactions:
E
H2O ! •OH þ Hþ þ e�

H2O2 þ Fe2þ þ Hþ ! Fe3þþ •OH þ H2O

In the classical Fenton process, the Fenton’sreagent (H2O2 + Fe2+) is added externally to thereaction medium. This leads to the formation ofFe(OH)3 sludge and promotes wasting reactionsthat decrease process efficiency. EF process has agreat advantage to generate in situ H2O2 (fromtwo-electron reduction of O2) and regenerateferrous iron ions (from initially added catalyst)as schematized on the following electrochemicalreactions:

O2 þ 2Hþ þ 2 e� ! H2O2

Fe3þ þ e�Fe2þ

There is no accumulation of Fe3+ and otherreagents (H2O2 and Fe2+), so sludge formationoccurs and parasitic reactions are minimized toenhance mineralization (transformation oforganics to CO2 and water) efficiency.

In the case of using a high O2 evolution anodesuch as boron-doped diamond (BDD) filmanode, •OH are formed simultaneously at theanode as well as in the bulk solution from theelectrogenerated Fenton’s reagent. The oxidationpower and mineralization efficiency are extremelyhigh (Dirany et al. 2010).

The •OH thus formed by the process in a con-tinuous and catalytic way will react with theorganic pollutants present in the medium until

their mineralization, i.e., transformation to CO2,H2O, and inorganic ions.

This EF process has been shown to be moreefficient and cost-effective than some widelyAOPs such as Fenton oxidation and ozonationfor the treatment of organic pollutants species(Brillas et al. 2009; Oturan and Aaron 2014). Itwas successfully applied to the removal of toxicand persistent organic pollutants from water suchas synthetic dyes, chlorinated aromatics, chloro-/nitrophenol herbicides, insecticides, fungicidesand other pesticides, and pharmaceutical and per-sonal care products. The results of these studieshave shown that the EF process using a largesurface area carbon-felt cathode is a very promis-ing technology due to its simplicity, low cost, andoutstanding performance, which is mainly due tothe quick and efficient cathodic regeneration ofthe Fe2+ catalyst.

References

Brillas E, Sirés I, Oturan MA (2009) Electro-Fenton processand related electrochemical technologies based onFenton’s reaction chemistry. Chem Rev 109:6570–6631

Dirany A, Sirés I, Oturan N, Oturan MA (2010) Electro-chemical abatement of the antibiotic sulfamethoxazolefrom water. Chemosphere 81:594–602

Oturan MA, Aaron JJ (2014) Advanced oxidation pro-cesses in water/wastewater treatment: principles andapplications. A review. Crit Rev Environ Sci Tech.doi:10.1080/10643389.2013.829765

Panizza M, Cerisola G (2009) Direct and mediated anodicoxidation of organic pollutants. ChemRev109:6541–6569

Electrofiltration


Electrofiltration represents a modification ofdead-end membrane micro- or ultrafiltration. Ittargets a significant reduction of the filtration

Electrofiltration,Fig. 1 Schematic sketch ofelectrofiltration principle,Fw stands for driving forcedue to the friction betweenthe particle and flowingsolvent molecules and Festands for driving forceresulting from action of theapplied electric field E onthe particle carryingelectrical charge

644 Electrolyzers

time and focuses especially on the filtration and/orconcentration of colloidal substances that other-wise rapidly build up a deposit of colloidal parti-cles on the surface of the membrane, whichstrongly hinders permeation of the fluid phase.The basic principle is that colloidal particlesusually carry an electric charge. Hence, by apply-ing an appropriate electric field, colloidal particlescan be moved in the direction opposite to the fluidflow, thus keeping the surface of the filtratingmembrane free of the deposit (Henry et al.1977). A schematic sketch of this arrangement isshown in the Fig. 1.

As the filtrating membrane remains unimpededby a deposit of colloidal particles, the pressuredrop across it remains relatively low. In contrast,the separator covered by the layer of colloidalparticles attracted by the electric field does notpermit a significant fluid flow. This results in areduction of the shear stress forces in the depos-ited film of separated colloid. These facts makeelectrofiltration especially promising for the sep-aration of biotechnology-derived products, thereason being that such products are typically sen-sitive to high shear stress forces while at the same

time they are electrically charged. The mild con-ditions of electrofiltration thus enable their prop-erties to be preserved during the process of theirseparation (Gözke and Posten 2010).

References

Henry jr. JD, Lawler LF, Kuo CHA (1977) A solid/liquidseparation process based on cross flow andelectrofiltration. AIChE Journal 23:851

Gözke G, Posten C (2010) Electrofiltration of Biopoly-mers. Food Eng. Rev. 2:131

Electrolyzers

Antonino Salvatore AricoCNR-ITAE Institute, Messina, Italy

One of the main processes occurring in anelectrolyzer device is the water electrolysis. Elec-trolysis of water is the dissociation of water mol-ecules into hydrogen and oxygen gases. For this

«H2O H2 + 0.5O2 Erev° = 1.23 V

Electron FlowH2O H2O

O2 H2

Solid Polymer Electrolyte

H+

H+

H+

ANODE

CATHODE

AnodeH2O 2H+ + 2e- + 0.5O2Erev° = 1.23 V vs. RHE

Metal Metal OxidesOxides

«Erev° = 0.00 V vs. RHE

Cathode

Pt/CPt/C

2H+ + 2e- « H2

Electrolyzers, Fig. 1 Principle of operation of a PEMwater electrolysis cell

Electrolyzers 645

E

process, in the presence of liquid water at 298 Kand 1 bar, DG� is 237 kJ mol�1 (corresponding to~1.23 V), DS� is 163 J mol�1 K�1

(TDS�~0.25 V), whereas DH� is 286 kJ mol�1.The thermoneutral potential at which this reactionoccurs in the absence of external heat supply isEth,DH = 1.48 V (upper heating value3.54 kWh�Nm�3 H2) (Millet et al. 2011). Ifsteam is fed to the device, the reaction enthalpyis reduced by ~40 kJ mol�1 corresponding to thevaporization enthalpy. Water electrolysis is tradi-tionally carried out in alkaline media with severalcommercial electrolyzers available on the market.Water electrolyzers using a solid polymer electro-lyte are less common and generally use expensivematerials such as noble metal electrocatalysts andNafion membranes (Barbir 2005; Siracusanoet al. 2010). Polymer electrolyte membrane(PEM) electrolyzers represent a viable alternativeto alkaline electrolyzer using KOH or NaOH aselectrolytes for hydrogen generation. The advan-tages of SPE water electrolyzers especially con-cern with increased safety, high energy density,and low maintenance. In the PEM waterelectrolyzer, water is usually supplied to theanodic compartment where oxygen evolutionoccurs, whereas hydrogen is produced at the cath-ode by protons transported through the protonicmembrane (Fig. 1). The electrodes are usuallycomposed of a platinum electrocatalyst for hydro-gen evolution, whereas metal oxides (e.g., IrO2,RuO2, etc.) are used for the anode due to theirenhanced activity and stability than Pt for thisreaction (Marshall et al. 2007; Siracusanoet al. 2010). The performance of an SPEelectrolyzer is strongly related to the characteris-tics of the membrane and electrode assembly(MEA) where the electrochemical reactions takeplace at triple-phase boundary. Therefore, theinterface between solid polymer electrolyte andelectrocatalyst layers should be characterized by asuitable extension; furthermore, the contact resis-tance between the catalytic layer and the mem-brane should be as low as possible. Generally,Nafion®membrane is used as conducting polymerelectrolyte in PEM electrolyzer systems. How-ever, low levels of H2 and O2 crossover are nec-essary for PEMWE application due to the high-

pressure operation that may reach 50–100 bars.Thus, a proper thickness is necessary for the poly-mer electrolyte separator (around 100 mm). Forhigh-pressure operation in PEM electrolyzers,reinforced PFSA membranes provide a propercombination of good conductivity and highmechanical strength.

References

Barbir F (2005) PEM electrolysis for production ofhydrogen from renewable energy sources. Sol Energy78:661

Marshall A, Børresen B, Hagen G, Tsypkin M, Tunold R(2007) Hydrogen production by advanced protonexchange mebrane (PEM) water electrolysers –Reduced energy consumption by improvedelectrocatalysis. Energy 32:431

Millet P, Mbemba N, Grigoriev SA, Fateev VN,Aukauloo A, Etiévant C (2011) Electrochemical per-formances of PEMwater electrolysis cells and perspec-tives. Int J Hydrog Energy 36:4134

Siracusano S, Baglio V, Di Blasi A, Briguglio N, Stassi A,Ornelas R, Trifoni E, Antonucci V, Arico AS(2010) Electrochemical characterization of single celland short stack PEM electrolyzers based on a nanosizedIrO2 anode electrocatalyst. Int J Hydrog Energy35:5558

646 Electromembrane

Electromembrane

Gérarld PourcellyInstitut Europeen des Membranes, CC 047,Universite Montpellier II, Place Eugene Bataillon,Montpellier, France

Electromembrane or “charged membrane” standsfor ion-exchange membrane [IEM]. They are usedin a number of processes which are rather differentin their basic concept, their practical applications,and their technical relevance (Strathmann 2004).All IEM separation processes are based on thesame fundamental principle which is the couplingof the transport of electrical charges, i.e., an elec-trical current with a transport of mass, i.e., cationsor anions, through a permselective membrane dueto an externally applied or internally generatedpotential gradient. There are two types of IEM:(i) monopolar and (ii) bipolar membranes.

Monopolar membranes are either cation-exchange membranes which contain negativelycharged groups fixed to the polymer matrix oranion-exchange membranes which contain posi-tively charged groups fixed to the polymer matrix.

Counter-io

Counter-ion

a

b

Electromembrane,Fig. 1 (a) Cation-exchange membrane with ahomogeneous structure; (b)ion-exchange membranewith a heterogeneousstructure prepared from anion-exchange resin powderin a binder polymer (FromStrathmann 2010)

In a cation-exchange membrane, the fixed nega-tive charges are in electrical equilibrium withmobile cations (counterions) in the interstices ofthe polymer as shown in Fig. 1 (Strathmann2010). In this case, the mobile anions are referredto as coions. They are more or less excluded fromthe polymer matrix because of their electricalcharge which is identical to that of the fixed ions(Donnan exclusion (Donnan 1911)). Thus, theselectivity of an IEM results from the exclusionof coions from the membrane phase. The proper-ties of IEM are determined by different parame-ters such as the density of the polymer network,the hydrophobic/hydrophilic character of thepolymer matrix, the nature and the ratio of fixedion-exchange groups, the cross-linking ratio, etc.

The most desired properties of IEM are (i) highchemical and thermal stabilities, (ii) high mechan-ical and dimension stabilities, (iii) highpermselectivity, (iv) low electrical resistance,(v) and a low cost.

Bipolar membranes (BPMs) are composed oftwo layers of ion exchangers joined by a hydro-philic junction (Pourcelly et al. 2009). The diffu-sion of water from both sides of the BPM allowsits dissociation under the electrical field to gener-ate protons and hydroxyl ions, which further

n pathway

pathway

Counter-ion Co-ion Fixed ion

Polymer matrix

Solution filled gaps

Ion-exchange resin Binder polymer

Electromembrane, Fig. 2 Principle of a bipolar membrane. Left hand: water dissociation under electrical field. Righthand: the two ion-exchange layers bearing fixed anion- or cation- exchange groups

Electromembrane Processes 647

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migrate from the junction layer through thecation- and anion-exchange layers of the BPM asdepicted in Fig. 2. The requirements for suitabilityof BPM include that for monopolar membranesbut also an experimental potential to achieve thewater-splitting capability as close as possible asthe theoretical value equal to 0.83 V at 25 �C.

Nowadays, superior styrene-divinylbenzenecopolymer membranes can be easily purchased,perfluorinated membranes with great chemicalstability are on the market, and BPM with anindustrial-scale lifetime (>20,000 h) is available.

References

Donnan FG (1911) Theory of membrane equilibrium andmembrane potential in the presence of non-dialysingelectrolyte. Z Electrokem Angew Phys Chem17:572–581

Pourcelly G, Bazinet L (2009) Developments ofBPM technology in food and bio-industries. In:Pabby AK, Rizvi SSH, Sastre AM (eds) Handbook ofmembrane separations, CRC Press, Boca Raton,pp 581–634

Strathmann H (ed) (2004) In: Ion exchange membraneseparation processes. Membrane technologies series,Elsevier, Amsterdam

Strathmann H (2010) Electrodialysis: a mature technologywith a multitude of new applications. Desalination264:268–288

Electromembrane Processes

Gerald PourcellyInstitut Europeen des Membranes, CC 047,Universite Montpellier II, Place Eugene Bataillon,Montpellier, France

Electromembrane processes involve ion-exchangemembranes. They can be divided into threetypes: (i) Separation of components such assalts or acids and bases from electrolyte solu-tions. In this case, the driving force for the iontransport across the membrane is an electricalpotential as in electrodialysis or a concentrationgradient as in both diffusion dialysis and Donnandialysis. (ii) The second type of processesinvolves an electrochemical reaction producingchemicals such as chlorine, acids and bases, ororganic and inorganic compounds. The mostknown process of this type is the chlorine-alkaliproduction where the ion-exchange membrane isthe key component. (iii) The third type ofion-exchange membrane processes involves theconversion of chemical into electrical energy andvice versa. The solid polymer electrolyte fuel cellis the most significant application (Strathmann2004).

CONCENTRATE (OUTLET)

DILUATE (OUTLET)

CONCENTRATE (INLET)

DILUATE (INLET)

CATHODE

ELECTROLYTE

ANODE

CrD C D C D C D C D DC

Cr

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Cr

Cr

Cr

Cr

Cr

Cr

Cr

Cr

Cr

Electromembrane Processes, Fig. 1 Scheme of a conventional two-compartment electrodialysis stack (C): concen-trate, (D): diluate compartments

648 Electromembrane Processes

(i): In dialysis, the driving force for the com-ponent transport is a difference of concentrationbetween the two solutions separated by a mem-brane. In diffusion dialysis and Donnan dialysisthe driving force is also a concentration gradientbut the membrane is carrying anions or cationsaccording to its permselectivity. Diffusion dialysiswith anion exchange membranes is used on a largescale to recover acids from pickling solutions(Kobuchi et al. 1987). Donnan dialysis is usedfor water softening or for recovering organicacids from their salts (Kliber and Wisnieskwi2011).

Electrodialysis (ED) is used to transport saltions from one solution through ion-exchangemembranes to another solution under the influ-ence of an applied electric potential difference.The elementary cell consists of a feed (diluate)compartment and a concentrate (brine) compart-ment formed by an anion exchange membraneand a cation exchange membrane placed betweentwo electrodes. In almost all practical ED pro-cesses, multiple ED cells (few hundreds) arearranged into a configuration called an ED stackFig. 1.

Conventional ED is used mainly for desalina-tion of saline solutions (chemicals) or milk whey(food industry). It can be combined with bipolarmembranes (bipolar membrane electrodialysis)

and used to produce acids and bases from theircorresponding salts, Fig. 2. This process is eco-nomically very attractive and has several interest-ing potential applications mainly in food industry,fine chemicals, or biotechnologies (Pourcellyet al. 2009).

(ii): The most known membrane electrolysisprocesses are for the production of chlorine andcaustic soda (Pletcher and Walsh 1990) or specialorganic compounds (Sata 1991). In the chlorine-alkali process, the cells are arranged in asimilar way as in an electrodialysis stack but intwo different configurations using monopolar orbipolar electrodes. Due to the severe operatingconditions (80 �C, 35 wt% NaOH, wet chlorine),the membrane is a composite structure ofsulfonated and carboxylated perfluorinatedreinforced polymers.

(iii): Ion-exchange membranes are used todayalso as key components in energy storage andconversion systems such as batteries and fuelcells. A fuel cell is an electrochemical reactor inwhich energy is converted into electrical energy.Generally fuel cells are fed with hydrogen whichis transformed into protons at a catalytic anodewith electrons. Protons then migrate across a pro-ton conducting membrane and combine with oxy-gen to produce water at the catalytic cathode(Lucia 2014).

Electromembrane Processes, Fig. 2 Electrodialysis with bipolar membrane. Scheme of a three-compartmentcell

Electroosmosis 649

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References

Kliber S, Wisnieskwi JA (2011) Removal of bromate andassociated anions from water by Donnan dialysis withanion exchange membranes. Desalin Water Treat35:158–163

Kobuchi Y, Motomura H, Noma Y, Hanada F (1987)Application of ion exchange membranes to recoveracids by diffusion dialysis. J Membr Sci 27:173–179

Lucia U (2014) Overview on fuel cells. Renew SustainEnergy Rev 30:164–169

Pletcher D, Walsh FC (1990) Industrial electrochemistry.Chapman & Hall, London

Pourcelly G, Bazinet L (2009) Developments of BPMtechnology in food and bio-industries. In: Pabby AK,Rizvi SSH, Sastre AM (eds) Handbook of membraneseparations. CRC Press, Boca Raton, pp 581–634

Sata T (1991) Ion exchange membranes and separationprocesses with chemical reactions. J Appl Electrochem21:283–294

Strathmann H (2004) In: Ion exchange membrane separa-tion processes. Membrane technologies series.Elsevier, Amsterdam

Electronic and Ionic Conductivity

▶Mixed Conducting Membranes

Electroosmosis


Electroosmosis is the term used for the convectiveflow of a solution containing charge carriers, e.g.,in the form of dissociated salts, through thepores or structure of a material with anelectrically charged internal surface. In the fieldof membrane processes, this phenomenon is typ-ically discussed in connection with the utilizationof ion-selective membranes under current load.However, it can potentially occur in any mem-brane material. The schematic sketch of the prin-ciple of electroosmotic flux initiation in anion-selective membrane pore (void space) isshown in Fig. 1.

The driving force of electroosmotic flux is theelectric field. Flux velocity is directly proportionalto the hydraulic permeability of a membrane and

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Electroosmosis, Fig. 1 The schematic sketch of a prin-ciple of electroosmotic flux initiation in the membrane withan electric charge carriers fixed on the pore (void fraction)wall

650 Electroosmosis, Overview of

indirectly proportional to the viscosity of the porefluid. The latter parameter represents an excess offree charge carriers of one sign in the bulk of thepore fluid. An excess of one-sign charge carriers ispossible on account of an intrinsic property of anion-selective membrane, i.e., the presence ofcharge-carrying functional groups in the mem-brane structure. These ion groups are bonded bya covalent bond to the backbone of the membranematerial. They are thus fixed and immobilized inthe structure. In the case of conventional mem-brane materials, the role of fixed charge can becarried out by ions specifically adsorbed on thepore walls or along the backbone of membranematerials. The charge carried by functional groupsand/or adsorbed in the internal structure of themembrane has to be compensated by an excesscharge of opposite sign present in the pore fluid.The electroosmotic convective flow of the porefluid is induced by friction force between thesolvated mobile charge carriers (ions) movingunder the action of the electric field and the sur-rounding liquid phase molecules (Squires andBazant 2004).

Electroosmotic flux represents one of themechanisms of mass transfer through anion-selective membrane. Its significance increaseswith increasing current load and, thus, the inten-sity of the electric field applied across the mem-brane (Fila and Bouzek 2008).

Cross-References


References

Fila V, Bouzek K (2008) The effect of convection in theexternal diffusion layer on the results of a mathematicalmodel of multiple ion transport across an ion-selectivemembrane. J Appl Electrochem. 38:1241

Squires TM, Bazant MZ (2004) Induced-charge electro-osmosis. J Fluid Mech 509:217

Electroosmosis, Overview of

Dianne Wiley1 and Gustavo Fimbres Weihs21School of Chemical and BiomolecularEngineering, The University of Sydney, Sydney,NSW, Australia2Cátedras CONACYT, Instituto Tecnológico deSonora, Cd. Obregón, Sonora, Mexico

Synonyms

Electroosmosis; Electroendosmosis; Electro-osmosis; Electroosmotic flow; EOF

Electro-osmosis is the movement of liquid inresponse to an applied electric field across a con-duit such as a membrane, capillary tube,microchannel, or porous material.

The application of an external electric fieldnear a solid/liquid interface may result in themotion of liquid with respect to an adjacentcharged surface. These associated effects areknown as electrokinetic phenomena (Hunter1981), first observed by Reuss in 1808 (Reuss1809). Electrokinetic phenomena includeelectro-osmosis, electroacoustics, electrophoresis,diffusiophoresis, streaming potential, zeta poten-tial, and sedimentation potential. The earliestexperiments on electrokinetic phenomena focusedon electro-osmosis because these are the easiest toconduct (Wall 2010). Early investigators includedWollaston, Porrett, Napier, Faraday, and Daniell.

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+ −

Electric Field

EDL

EDL

Membrane

Membrane

- - - - - - -- -

- - - - - - -- -

+++++++ +++++++ +++++++ +++++++ +++++++ +++++++ +++++++ ++++++++

++++++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++ +++++++++

--- --- --- --- --- --- ------ ---

--- --- --- --- --- --- ------ ---

+++++++++++ +++++++++++ +++++++++++ +++++++++++ +++++++++++ +++++++++++ +++++++++++ +++++++++++

+++++++++++++ +++++++++++++ +++++++++++++ +++++++++++++ +++++++++++++ +++++++++++++ +++++++++++++ ++++++++++++

Fluid

Electroosmosis,Overview of,Fig. 1 Schematicrepresentation of electro-osmosis in a membranepore, showing the effect ofthe electric field on theelectric double layer (EDL)and the resulting drag on thebulk fluid

Electroosmosis, Overview of 651

E

In electro-osmosis, the bulk fluid moves relativeto a charged surface due to an external electricfield.

The chemical equilibrium between a solid sur-face (such as a membrane) and a fluid containingcharged species typically leads to the interfaceacquiring a net fixed electrical charge (mostoften negative for membranes), characterized bythe zeta potential (z). The presence of co-ions(similarly charged ions) and counterions(oppositely charged ions) results in attractionand repulsion of ions in the vicinity of a chargedsurface. This phenomenon, coupled with the ran-dom thermal motion of the ions, creates an electricdouble layer (EDL) or Debye layer in the regionnear the interface. Electroneutrality is notmaintained within the EDL, because there is anexcess of counterions compared to co-ions inorder to neutralize the surface charge (Probstein2005). When an electric field is applied to thefluid, the net charge in the electrical double layeris induced to move by the resulting Coulombforce. The movement of ions drags bound solvent,causing bulk fluid motion due to momentumtransfer, as depicted in Fig. 1. The resulting flowis termed electro-osmotic flow. Electro-osmosiscan only occur if there are charged species in thefluid that can respond to the electric field(Jagannadh and Muralidhara 1996).

Electro-osmosis can be described mathemati-cally using the momentum transport equation, asthe resulting body force due to an electric field ona charged fluid (Hu and Li 2007). The net chargedensity of the fluid (re) can be related to the

electric potential (f) and the permittivity of thefluid (e) by Poisson’s equation:

e∇2f ¼ �re (1)

Under negligible convective or electrophoretic-induced flow, the charge density follows aBoltzmann distribution (Hunter 1981), and thecharge density can be obtained by solving thenonlinear Poisson–Boltzmann equation:

∇2f� k2sinh fð Þ ¼ 0 (2)

where the constant k depends on the ionic com-position (Russel et al. 1989). Although solutionsto Eq. 2 can only be obtained numerically forcomplex geometries, it can be linearized usingthe Debye–H€uckel approximation, which leadsto the following electric potential distribution:

f ¼ zexp�y

lD

� �(3)

where y is the distance from the surface, andlD = 1/k is the Debye length.

In membrane systems, electro-osmosis can beapproximated through the Helmholtz–Smo-luchowski (HS) velocity equation:

us ¼ � ezEx

m(4)

The HS approximation described by Eq. 4 is anartificial slip velocity implemented on a charged

652 Electroosmosis, Overview of

surface. It can be used to simulate the effect ofelectro-osmosis on the velocity in a conduit at theouter edge of the EDL (Probstein 2005). Thisequation implies that the electro-osmotic flowvelocity is proportional to the applied electricfield. Other equations that are relevant to electro-osmosis under different conditions include theH€uckel–Onsager equation and the Henryequation.

Most common applications of electro-osmosisare in capillary electrophoresis, capillaryelectrochromatography, and microfluidic devicesin fields as diverse as biophysics, dewatering,geomechanics, medicine, microchips, oil and gasproduction, and separations including in mem-brane separation. There are a number of importantapplications in membrane technology. At the fun-damental level, electro-osmosis can be used todetermine the zeta potential of membranes(Bowen and Clark 1984). It can also be beneficialfor the operation of membrane separation sys-tems, and has found numerous applications inmicrofiltration and ultrafiltration processes(Bowen 1993), in particular to do with the miti-gation of membrane fouling. This is because sur-face activity and membrane charge, twophenomena closely related to membrane fouling,cannot be influenced through hydrodynamicforces such as increased shear. Electro-osmosiscan be used for membrane cleaning and restora-tion (Bowen et al. 1989), for backwashing(Bowen and Sabuni 1994), for releasing filtercakes in dead-end membrane processes (Bowenand Ahmad 1997), and for enhancing flux (Sarkarand De 2010).

Although the use of DC electric fields in mem-brane separation processes has been in practice forseveral decades, anode corrosion is a significantproblem that has discouraged its wider application(Jagannadh and Muralidhara 1996). Nevertheless,the development and availability of corrosion-resistant materials has enabled recent develop-ments and commercialization of these techniques.Electro-osmosis has also found applications inRO processes, leading to improved recoverythrough membrane backwashing (Spiegler andMacleish 1981) or by increasing the level ofmixing in the boundary layer (Liang et al. 2014).

Cross-References

▶Backwashing▶Electrical Double Layer▶Electroosmotic Drag in Membranes▶Membrane Fouling▶ Surface Charge Density▶Zeta Potential

References

Bowen WR (1993) Electrochemical aspects ofmicrofiltration and ultrafiltration. In: Howell JA,Sanchez V, Field RW (eds) Membranes inbioprocessing: theory and applications. Springer, Dor-drecht, pp 265–291

Bowen WR, Ahmad AL (1997) Pulsed electrophoreticfilter-cake release in dead-end membrane processes.AIChE J 43(4):959–970

Bowen WR, Clark RA (1984) Electro-osmosis at micropo-rous membranes and the determination of zeta-potential. J Colloid Interface Sci 97(2):401–409

BowenWR, Sabuni HAM (1994) Electroosmotic membranebackwashing. Ind Eng Chem Res 33(5):1245–1249

BowenWR,KingdonRS, Sabuni HAM (1989) Electricallyenhanced separation processes: the basis of in situintermittent electrolytic membrane cleaning (iiemc)and in situ electrolytic membrane restoration (iemr).J Membr Sci 40(2):219–229

Hu G, Li D (2007) Multiscale phenomena in microfluidicsand nanofluidics. Chem Eng Sci 62(13):3443–3454

Hunter RJ (1981) Zeta potential in colloid science: princi-ples and applications. Academic, London

Jagannadh SN, Muralidhara HS (1996) Electrokineticsmethods to control membrane fouling. Ind Eng ChemRes 35(4):1133–1140

Liang YY, Fimbres Weihs GA, Wiley DE (2014) Approx-imation for modelling electro-osmotic mixing in theboundary layer of membrane systems. J Membr Sci450:18–27

Probstein RF (2005) Physicochemical hydrodynamics: anintroduction, 2nd edn. Wiley, New York

Reuss FF (1809) Sur un novel effet de l’électricitégalvanique. Mémoires de la Société Impériale desNaturalistes de Moskou 2:327–337

Russel WB, Saville DA, Schowalter WR (1989) Colloidaldispersions. Cambridge University Press, Cambridge,UK

Sarkar B, De S (2010) Electric field enhanced gel con-trolled cross-flow ultrafiltration under turbulent flowconditions. Sep Purif Technol 74(1):73–82

Spiegler KS, Macleish JH (1981) Molecular (osmotic andelectro-osmotic) backwash of cellulose acetatehyperfiltration membranes. J Membr Sci 8(2):173–192

Wall S (2010) The history of electrokinetic phenomena.Curr Opin Colloid Interface Sci 15:119–124

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Electroosmotic Drag in Membranes 653

Electroosmotic Drag Flux

▶Electroosmotic Drag in Membranes

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Electroosmotic Drag in Membranes

Dianne Wiley1 and Gustavo Fimbres Weihs21School of Chemical and BiomolecularEngineering, The University of Sydney, Sydney,NSW, Australia2Cátedras CONACYT, Instituto Tecnológico deSonora, Cd. Obregón, Sonora, Mexico

Synonyms

Electroosmotic drag flux; Electroosmotic watertransport; EOD

Electroosmotic drag in membranes refers to themovement of water or other electroneutral sol-vents through a membrane, associated with themovement of ions under the influence of an elec-tric field.

The zeta potential at the interface between amembrane and an electrolyte generally leads tothe creation of an electric double layer (EDL).When an electric field is applied to fluid withinan EDL, the net charge is induced to move by theresulting Coulomb force. The movement of ionsdrags bound solvent, causing bulk fluid motion

Drag

+

Electroosmotic Dragin Membranes,Fig. 1 Schematicrepresentation ofelectroosmotic drag in amembrane pore. Theelectric field causes thedissolved ions to experiencea net force, and these ionstransfer momentum to thesolvent molecules throughviscous drag

due to momentum transfer (viscous drag). Thisbulk fluid motion due to an external electric fieldis termed electroosmosis (De Groot 1966) and canoccur in any fluid with a net charge, including inmembrane channels or within membrane pores(Fig. 1).

Electroosmotic drag (EOD) is the underlyingmechanism responsible for solvent transport in anumber of membrane techniques including:

• EOD in electrodialysis (ED), often called“water transport,” is one of the reasons forlosing current density (Indusekhar andKrishnaswamy 1965). ED has many industrialapplications including the production of chlo-rine, NaOH, and other inorganic acids andbases (de Groot et al. 2011).

• EOD of water in proton exchange membranes(PEMs, also referred to as polymer electrolytemembranes), which leads to water manage-ment issues (Zawodzinski et al. 1995).

• EOD of methanol in direct methanol fuel cells(DMFCs), leading to methanol crossover(Heinzel and Barragán 1999).

• In vanadium redox batteries (VRBs), EOD cancause vanadium crossover and capacity loss(Agar et al. 2013).

• EOD in electroosmotic pumps (EOPs) is usedto generate flow or pressure (Wang et al. 2006).EOPs can be used to aid with the hydration ofPEMs for fuel cells.

• EOD is a mechanism for producinganomalous flux through charged membranes(e.g., NF membranes) due to the selective

−

Membrane

Membrane

Electric Field+

Drag

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654 Electroosmotic Flow

passage of one type of charged ion over theother, leading to the axial rearrangement of theEDLs (Sasidhar and Ruckenstein 1982).

EOD is of particular importance in PEMs suchas Nafion® and sulfonated polyetherketones, usedin fuel cells and VRBs. Water management criti-cally impacts the performance of PEMs for fuelcells, as their proton conductivity increases withhydration. Too little water can dry the membrane,leading to a reduction in conductivity and conse-quently lower cell performance. Too much watercan flood the membrane and lead to cathodeflooding problems. In addition to the generationof water at the cathode catalyst layer due to theelectrochemical reaction, water is also transportedthrough the membrane (Lee et al. 2008). Thepermeate flux of water in a PEM is governed bytwo main mechanisms: (a) the osmotic flux due toa chemical potential gradient across the mem-brane, which is influenced by both the activity ofthe solution (osmotic pressure) and the hydro-static pressure, and (b) the electroosmotic fluxdue to the drag induced by the proton flux (Renand Gottesfeld 2001).

In order to optimize water concentration in afuel cell membrane, it is important to quantifyelectroosmotic drag. The electroosmotic waterflux can be related to the proton flux throughthe membrane by means of the electroosmoticdrag coefficient (psdrag), which is defined asthe number of water molecules transportedthrough the membrane per proton as the chemicalpotential gradient of water tends to zero (Iseet al. 1999).

Cross-References

▶Direct Methanol Fuel Cell (DMFC)▶Electrical Double Layer▶Electrodialysis▶ Polymer Electrolyte Membrane Fuel Cell(PEMFC)

▶ Proton-Exchange Membranes for Fuel Cells▶Zeta Potential

References

Agar E, Knehr KW, Chen D, Hickner MA, Kumbur EC(2013) Species transport mechanisms governing capac-ity loss in vanadium flow batteries: comparing Nafion®

and sulfonated Radel membranes. Electrochim Acta98:66–74

De Groot SR (1966) Thermodynamics of irreversible pro-cesses, 4th edn. North-Holland, Amsterdam

de Groot MT, de Rooij RM, Bos AACM, Bargeman G(2011) Bipolar membrane electrodialysis for the alka-linization of ethanolamine salts. J Membr Sci378(1–2):415–424

Heinzel A, Barragán VM (1999) A review of the state-of-the-art of the methanol crossover in direct methanolfuel cells. J Power Sources 84(1):70–74

Indusekhar VK, Krishnaswamy N (1965) Diffusion effectduring electrodialysis with ion-exchange membranes.J Appl Polym Sci 9(7):2631–2632

Ise M, Kreuer KD, Maier J (1999) Electroosmotic drag inpolymer electrolyte membranes: an electrophoreticNMR study. Solid State Ion 125(1–4):213–223

Lee PH, Han SS, Hwang SS (2008) Three-dimensionaltransport modeling for Proton Exchange Membrane(PEM) fuel cell with micro parallel flow field. Sensors(Basel) 8(3):1475–1487

Ren X, Gottesfeld S (2001) Electro-osmotic drag of waterin poly(perfluorosulfonic acid) membranes.J Electrochem Soc 148(1):A87–A93

Sasidhar V, Ruckenstein E (1982) Anomalouseffects during electrolyte osmosis across chargedporous membranes. J Colloid Interface Sci85(2):332–362

Wang P, Chen Z, Chang H-C (2006) A new electro-osmoticpump based on silica monoliths. Sens ActuatorsB 113(1):500–509

Zawodzinski TA, Davey J, Valerio J, Gottesfeld S (1995)The water content dependence of electro-osmoticdrag in proton-conducting polymer electrolytes.Electrochim Acta 40(3):297–302

Electroosmotic Flow


Electroosmotic Water Transport


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Electrophoresis 655

Electrooxidation

Marc CretinInstitut Européen des Membranes, ENSCM-UM-CNRS (UMR 5635), Université de Montpellier,Montpellier Cedex 5, FranceNational School of Chemistry of Montpellier,University of Montpellier, Montpellier, France

E

Electrooxidation is a direct anodic oxidation tech-nique to remove biorefractory pollutants by directelectrolysis in which pollutants exchange elec-trons with the anode surface without intermediatesubstances. It differs from indirect anodic oxida-tion. Indeed by indirect anodic oxidation, pollut-ants are oxidized by •OHs radicals generated bywater oxidation on a high O2 evolution anodesuch as boron-doped diamond (BDD).

Direct anodic oxidation is possible using noblemetals (Pt and Pd), metal oxide (IrO2, PbO2), orcarbon-based (boron-doped diamond) anodes,before oxygen evolution. The first stage is thepollutant adsorption on the anode surface. Thesecond stage is the oxygen (as a source of elec-trons) transfer from water to the organic pollutantusing electric energy (i.e., anodic polarization)through the so-called electrochemical oxygentransfer reaction (EOTR). A typical example ofEOTR often cited in the literature (Panizza andCerisola 2009) to illustrate direct electrooxidationis anodic incineration of phenol as follows:

C6H5OHþ 11H2O ! 6CO2 þ 28Hþ þ 28e�

The complete phenol oxidation to CO2 (i.e., min-eralization) is obtained at the anode, and protonsliberated in this reaction are reduced at the cathodeto hydrogen. The pollutant mineralization ratedepends mainly on the nature of the anodicmaterial, the choice of the anodic potential(fixed before oxygen evolution), and chemicaland physical parameters such as electrolyte andpollutant composition and concentration andtemperature.

The main limitation of electrooxidation is thedecrease of the electrocatalytic activity of theelectrode because of the fast formation of a poly-mer layer from oxidation intermediates on theanode surface. Film formation depends on theadsorption properties of the electrode surface,the concentration and the nature of organic com-pounds, their degradation intermediates, the pres-ence of oxygen, etc. Then electrodes presenting asurface with weak adsorption properties are pro-moted to gain in stability (Panizza and Cerisola2003). Nevertheless, in anaerobic conditions, theformation of the polymer fouling layer remainsthe limiting factor of the mineralization.

To overcome this poisoning effect, anode mustbe polarized over oxygen evolution to promotewater discharge, leading to hydroxyl radical(•OH) formation on specific electrode like boron-doped diamond. The electrochemical formation ofthis powerful oxidant avoids then polymer foulinglayer formation and then the decrease of theelectrocatalytic activity of the electrode. Moredetails can be found concerning this electrochem-ical advanced oxidation process (EAOP) in theentry “indirect anodic oxidation.”

References

Panizza M, Cerisola G (2003) Influence of anode materialon the electrochemical oxidation of 2-naphthol – part 1.Cyclic voltammetry and potential step experiments.Electrochim Acta 48:3491–3497

Panizza M, Cerisola G (2009) Direct and mediated anodicoxidation of organic pollutants. Chem Rev109:6541–6569

Electrophoresis

Catherine CharcossetUniversité Lyon 1, Lyon, Villeurbanne, France

The term electrophoresis refers to the motion ofsuspended particles in an applied electric field.Among separation techniques, electrophoresis is

656 Electrophoresis

widely used in research and development andquality control in disciplines such as biochemis-try, immunology, genetics, and molecular biology(Westermeier 2001). Electrophoresis is based onthe differential migration of charged species in asemiconductive medium under the influence of anelectric field. Separation of many different kindsof species including proteins, DNA, nucleotides,drugs, and many other biochemicals is obtainedupon differences in size, charge, and hydropho-bicity. The technique was first reported in 1937 byArne Tiselius who won the Nobel Prize in Chem-istry in 1948 for the separation of different serumproteins by a method called “moving-boundaryelectrophoresis.” Since then, a number ofimproved techniques have been introduced suchas gel electrophoresis, capillary electrophoresis,and two-dimensional electrophoresis.

Gel electrophoresis uses an electric currentpassed through an agarose or polyacrylamide gel(SDS-PAGE) to separate the molecules in a sampleon the basis of their differences in molecular sizeand charge. As the sample migrates in the gel inresponse to the electric current, the smaller speciesmove more quickly than the larger species, whichresults in a distinct banded pattern in the gel. Thisbanded pattern may be visualized via the applica-tion of staining agents, such as ethidium bromide,which reveals the gel bands under UV light, orsilver stain, which is typically used to detect pro-teins. The silver stain is compatible with massspectrometry techniques for further analysis of theprotein composition. Capillary electrophoresis(CE) involves a combination of both polyacryl-amide gel electrophoresis (SDS-PAGE) and high-performance liquid chromatography (HPLC)(Ahuja and Jimidar 2008). High voltages of500 V/cm or greater are generated within narrowcapillaries (20–200 mm). The high voltages causeelectroosmotic and electrophoretic movement ofbuffer solutions and ions, respectively, within thecapillary. Two-dimensional gel electrophoresis(2-D electrophoresis) separates species in twosteps, according to two independent properties. Ina common technique, the first dimension is isoelec-tric focusing, which separates proteins according totheir isoelectric points; the second dimension isSDS polyacrylamide gel electrophoresis

(SDS-PAGE), which separates proteins accordingto their molecular size. The method involves plac-ing the sample in gel with a pH gradient andapplying a potential difference across it.

Cellulose acetate membranes are other currentsupporting media for electrophoresis separation(Westermeier 2001). They are used for routine clin-ical analysis and related applications, as well as forthe analysis of molecules in physiological fluids.These membranes have large pores and thereforehave a low sieving effect on molecules. The elec-trophoretic separation is thus entirely based oncharge density. The matrix exerts little effect ondiffusion so that the separated zones are relativelywide and the resolution and limit of detection areais low. For these reasons, cellulose acetate mem-branes are often replaced by gel electrophoresis.

Other supporting membranes for electrophore-sis include Nafion membranes, a type of perfluor-osulfonic acid membrane, and cation-exchangemembranes, which are chemically resistant andconsist of a pore-structure cluster network (Fanget al. 2004). These membranes are widely used inthe field of chloralkali industry and in fuel cells.A Nafion membrane contains hydrophilic pores(10–20 Å and 50–60 Å in size) acting as verynarrow electrophoresis channels. The fixed-charge sites (�SO3

�) on the hydrophilic poresurface provide a strong charged background.Nafion membrane electrophoresis is a potentiallyattractive technique for the separation of smallorganic molecules like amino acids or ions.

Cross-References

▶ Ion-Exchange Membranes

References

Ahuja S, Jimidar M (2008) Capillary electrophoresismethods for pharmaceutical analysis. Academic,Amsterdam

Fang C, Wu B, Zhou X (2004) Nafion membrane electro-phoresis with direct and simplified end-column pulseelectrochemical detection of amino acids. Electropho-resis 25:375–380

Westermeier R (2001) Electrophoresis in practice, 3rd edn.Wiley-VCH, Weinheim

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Electrophoretic Deposition 657

Electrophoretic Deposition


E

Electrophoretic deposition (EPD), also calledelectrocoating, e-coating, cathodic electrodeposi-tion, or electrophoretic coating, is a simple andeffective technique for coating of charged parti-cles on substrates (Besra and Liu 2007). It hasseveral advantages including continuousprocessing, uniform deposition and control ofthe thickness, and morphology of a depositedfilm by adjustment of the deposition time andapplied potential. In EPD, charged powder parti-cles, dispersed or suspended in a liquid medium,are attracted and deposited onto a conductive sub-strate of opposite charge on application of a DCelectric field. There are two types of electropho-retic deposition (Fig. 1). When the particles arepositively charged, the deposition happens on thenegative electrode (cathode) and the process istermed cathodic electrophoretic deposition. Thedeposition of negatively charged particles on pos-itive electrode (anode) is called anodic electro-phoretic deposition. By suitable modification ofthe surface charge on the particles, any of the twomodes of deposition is possible. This technique isconvenient for stable suspensions containingcharged particles free to move when an electricfield is applied. Therefore, EPD can be applied toany material that is available as a fine powder(e.g., <30 mm particle size) or as a colloidal

a

−

ElectrophoreticDeposition,Fig. 1 Schematic ofelectrophoretic depositionprocess. (a) Cathodic EPDand (b) anodic EPD

suspension, includingmetals, polymers, ceramics,and glasses.

The fundamental mechanisms of EPD aredescribed in the literature mainly in the frameworkof the Derjaguin–Landau–Verwey–Overbeek(DLVO) theory and in relation to the distortion ofthe particle double layer under the application of aDC electric field (Corni et al. 2008). However,numerous other theories (flocculation by particleaccumulation, particle charge neutralization, elec-trochemical particle coagulation, electrical doublelayer distortion, and thinning mechanism) havebeen proposed to explain the particle interactionsand the kinetics of deposition.

Several examples of membrane preparationusing an EPD technique are reported. For exam-ple, EPD is used as a seeding method for makingzeolite membranes (Abdollahi et al. 2007). Zeo-lites are crystalline structures which possess uni-form and molecular-sized pores. Zeolitemembranes have a great potential in separationand catalysis processes owing to their uniquepore structures and adsorption properties andtheir high thermal, mechanical, and chemical sta-bility compared with polymeric membranes.Since zeolites are negatively charged particles,they can be effectively attracted to the substratesof positive charge. By the aid of EPD, an orientedcontinuous layer of nano-sized zeolite seeds isformed on the support and acts as nuclei for thenext step which is crystal growth under hydrother-mal situation.

The EPD technique is also used for the prepa-ration of membrane electrode assembly for fuelcell (Morikawa et al. 2004). A membrane

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++ −−

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658 Electrophoretic Mobility

electrode assembly can be prepared by EPD pro-cess onto a Nafion membrane. A suspensionconsisting of ethanol, carbon powders with Ptcatalyst, and Nafion polymer is used to obtain astable dispersed solution. The thickness of theprepared catalyst layer is controlled by EPD dura-tion or concentration of suspension. The cellobtained may present a higher electrochemicalperformance compared with ordinary cells.

Cross-References

▶Electrophoretic Painting▶ Fuel Cell Membrane▶Zeolite Membrane

References

Abdollahi M, Ashrafizadeh SN, Malekpour A (2007) Prep-aration of zeolite ZSM-5 membrane by electrophoreticdeposition method. Microporous Mesoporous Mater106:192–200

Besra L, Liu M (2007) A review on fundamentals andapplications of electrophoretic deposition (EPD). ProgMater Sci 52:1–61

Corni I, Ryan MP, Boccaccini AR (2008) Electrophoreticdeposition: from traditional ceramics to nanotechnol-ogy. J Eur Ceram Soc 28:1353–1367

Morikawa H, Tsuihiji N, Mitsui T, Kanamura K (2004)Preparation of membrane electrode assembly for fuelcell by using electrophoretic deposition process.J Electrochem Soc 151:A1733–A1737

Electrophoretic Mobility


When an electric field is applied across a givenmedium, charged solutes or particles suspended inthe electrolyte are attracted toward the electrodeof opposite charge. Viscous forces acting on thecomponent tend to oppose this movement. Whenequilibrium is reached between these two oppos-ing forces, the solutes or particles move withconstant velocity. The velocity is dependent on

factors such as the strength of electric field orvoltage gradient, the dielectric constant of themedium, the viscosity of the medium, and thezeta potential of the particles.

The electrophoretic mobility, m (m2/Vs), is theobserved electrophoretic velocity, v (m/s), dividedby electric field strength, E (V/m):

m ¼ v

E

The electrophoretic velocity is the distance ofmigration divided by time, also called velocityof migration. Mobilities are sometimes expressedwith a negative sign, because migration of thesolutes or particles generally occurs in the direc-tion opposite to the electrophoretic field (which istaken as reference).

The electrophoretic mobility of charged sol-utes, including proteins, is predicted using theDebye-H€uckel-Henry theory (O’Connoret al. 1996). This theory is valid only for sphericalnonconducting particles at low zeta potentials,with ions present in the electrical double layerbehaving as point charges. The Debye-H€uckel-Henry theory gives the mobility as

m ¼ ze

6p�rf krð Þ1þ krð Þ

where z is the net charge (dimensionless), e theelementary charge (C), r the particle radius (m), �the viscosity (kg m�1 s�1), and f(kr) is theHenry correction factor, which has valuesbetween 1 and 1.5. The Debye-H€uckel parameter,k, is defined by

k2 ¼ 2e2NAI

ekT

whereNA is the Avogadro’s constant (kmol�1), I isthe ionic strength (kmol m�3), e is the permittivity(J�1 C2 m�1), k is the Boltzmann’s constant(J K�1), and is T the temperature. O’Brien andWhite (1978) obtained a more rigorous solution tothe equations describing the electrophoreticmobility of a rigid spherical particle with a uni-form charge density. The predictions of this model

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coincide with those of the Debye-H€uckel-Henrytheory for systems with zeta potentials less than25 mV.

The electrophoretic mobility of proteins hasbeen determined through several microfiltrationmembranes under conditions of zero concentra-tion driving force and with negligible adsorptionon to the membranes occurring during the exper-iment (O’Connor et al. 1996). The initial mobilityof both proteins is close to free-solution values atthe same pH, after correction for the ionic strengthof the buffer. This effect is explained by the effectof reduced free area in the membrane which iscompensated by a corresponding increase in thepotential gradient. Data were also obtained formixtures of two proteins. Interactions occurwhen the two proteins are oppositely charged,changing the apparent mobilities from the freesolution values.

Cross-References

▶Electrical Potential▶Electrophoresis

References

O’Brien RW, White LR (1978) Electrophoretic mobility ofa spherical colloidal particle. J Chem Soc FaradayTrans 1:1607–1626

O’Connor AJ, Pratt HRC, Stevens GW (1996) Electropho-retic mobilities of proteins and protein mixtures inporous membranes. Chem Eng Sci 51:3459–3477

Electrophoretic Painting


Electrophoretic painting, also called electropho-retic deposition of paint, electrodeposition paint-ing, or E-coating, is an economical and corrosion-resistant technique for applying coatings to elec-trically conducting materials. It is widely used to

coat many industrial products such as automobilebodies and parts, tractors and heavy equipment,metal furniture, and beverage containers. The pro-cess is carried out with the use of anodes andcathodes in the painting tank. There are twotypes of electrophoretic painting: anaphoreticand cataphoretic painting. In anaphoretic paintingthe profiles will be the anode, and in cataphoreticpainting the profiles will be the cathode. In theelectrophoretic painting process, evolution ofoxygen gas will take place at the anode, andhydrogen gas will be evolved at the cathode.

The electrophoretic paint is continuously cir-culated to avoid settling of the paint solids andheat resulting from the pumping process as well asfrom the passage of electric current. Dry coatingthicknesses of the order of 20 mm are normallyapplied, using a voltage of 150–200 V for a timeof 1–2 min. After painting, the profiles are rinsedbefore stowing at temperatures of 180–200 �C for20–30 min. The electrophoretic paint may containa water-soluble acrylic-based paint, the resinbeing rendered soluble by the addition of suitableamines such as melamine. When current is passedthrough the paint, the resin and pigments willmigrate to the anode while the amine will bedischarged at the cathode. During paint depositionthe level of amine will increase, and this causes adecrease in paint deposition rate if allowed tocontinue. The most usual processes used forremoval of excess amine are ultrafiltration oftenin combination with reverse osmosis.

After electrophoretic painting, ultrafiltrationcontinuously treats the paint bath to produce per-meate needed for rinsing metal parts (Aganaet al. 2011). While permeate is used for rinsing,the mixture of paint and rinse water is returned tothe paint bath from the downstream rinse system.The volume in the paint bath remains constant,and the process leads to a closed loop cycleincluding a multistage rinse system. No wastewa-ter is produced and almost no deionized water isrequired for the purpose of rinsing. Different typesof membranes are available for various types ofpaint. The type and size of the membrane modulesare related to the needs of bath volume and char-acteristic parameters of the paint bath, such astotal solids, pH, and temperature.

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660 Electrospun Nanofiber Membrane for Biosensors

Cross-References

▶Electrophoretic Deposition▶ Permeate

References

Agana BA, Reeve D, Orbell JD (2011) Optimization of theoperational parameters for a 50 nm ZrO2 ceramic mem-brane as applied to the ultrafiltration of post-electrodeposition rinse wastewater. Desalination278:325–332

Electrospun Nanofiber Membranefor Biosensors

Michele Modesti, Carlo Boaretti andMartina RosoDepartment of Industrial Engineering, Universityof Padova, Padova, Italy

A biosensor is an analytical device comprisingthree main elements: a biological recognition ele-ment able to interact specifically with a target, atransducer able to convert a change in property ofthe solution or surface into a recordable signal,and a processing system which is able to amplifyand quantify the signal. The involved biomolecu-lar interactions in the bio-receptor/analytecomplex can be based on antibody-antigen inter-actions, enzymatic interactions, nucleic acid inter-actions, and cellular or analyte/synthetic receptorinteractions.

Biosensors are generally classified accordingto transduction modes and recognition elements(D’Orazio 2003; Rodriguez-Mozaz et al. 2004) infive principal transducer classes that are electro-chemical, optical, thermometric, piezoelectric,and magnetic. Moreover, electrochemical sensorsmay be subdivided into potentiometric, ampero-metric, or conductometric types (Thevenotet al. 2001). Biosensors represent nowadays theanswer to a pressing need of advanced solutionsfor sensitive detection of the physiologicalchemicals involved in clinical diagnosis.

Within this scenario, electrospinning is an eco-nomic and versatile technology for producingone-dimensional nanomaterials that possess highspecific surface area and high porosity. In thistechnology, an electrified jet of polymer solutionis obtained by applying a relatively high electrictension. If the viscoelastic forces on the polymersolution are sufficient to contrast jet breaking, anonwoven web of fibers ranging from few nano-meters to several microns is collected on a propersupport. Several reviews and books aboutelectrospinning (Bhardwaj and Kundu 2010;Ramakrishna et al. 2005; Wendorff et al. 2012)are available for having a better understanding ofthis technique. The majority of electrospun fibersare polymer-based or polymer-inorganic compos-ites. It has been shown they have a good biocom-patibility and consequently they represent a goodcandidate for biosensor fabrication especially as amatrix for protein or enzyme immobilization(Li et al. 2014).

Among biosensors, monitoring changes inextracellular glucose concentration is one of themost important ones because it is related to bothbrain disease and neurological disorders, and it isalso the key analyte for medical diagnostics andmanagement of diabetes. A complete and exhaus-tive review on glucose biosensor and all the basicprinciples of them can be found in the scientificliterature (Yoo and Lee 2010). The most commonglucose biosensors achieve specific recognition ofglucose by immobilization of the enzyme glucoseoxidase (GOD) on the surface of electrodes Wuand Fan (2013). The basic concept of the glucosebiosensor is based on the fact that the immobilizedGOD catalyzes the oxidation of b-D-glucose bymolecular oxygen, producing gluconic acid andhydrogen peroxide. Three general strategies areused for the electrochemical sensing of glucose:by measuring oxygen consumption, by measuringthe amount of hydrogen peroxide produced by theenzyme reaction, or by using a diffusible orimmobilized mediator to transfer the electronsfrom the GOD to the electrode (Yoo and Lee2010). At this regard, polyvinyl alcohol (PVA)nanofibers have been shown to be useful in pro-ducing an amperometric glucose biosensor (Renet al. 2006) which exhibited a rapid response time

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(1 s) and a higher output current (mA level) toglucose in the normal and diabetic level. Thelinear concentration response range from 1 to10 mM and the lower detection limit (0.05 mM)of the sensor can meet the demand in the detectionof the glucose in medical diagnosis.

In enzyme-based biosensors, the sensitivityand longevity of the biorecognition element arefunctions of the physical design and the enzymestability over time (Wilson and Gifford 2005).Moreover, temperature, pH, and other chemicalscan easily affect the activity and stability of theimmobilized enzyme as well as the immobiliza-tion chemistry and the microenvironment of theenzyme on the electrode (Marx et al. 2011).

In order to get an efficient enzyme encapsula-tion and the absence of interference from othercoexisting electroactive species, the biocompositebased on Prussian blue, chitosan, and polyvinylalcohol has been fabricated by electrodepositionand subsequent electrospinning in enzyme-friendly conditions. Prussian blue (PB), knownas “artificial peroxidase,” has been employed infabricating amperometric glucose biosensorsbecause of its well-known capabilities for enhanc-ing electron transport and excellent catalyticactivity toward H2O2 reduction at a lowoverpotential. The modified electrode has beenshown to exhibit such a rapid direct electron trans-fer rate that the glucose sensor exhibited excellentperformance, which includes wider linearity,lower detection limit, and good stability in opti-mized conditions (Wu and Yin 2013).

Another way for improving sensitivity andlifetime of the immobilized enzyme is the appli-cation of conducting polymers to bioelectronicsurfaces; the advantage of using such electrospunpolymers is related to their ability to increase thesignal-to-noise ratio (S/N), and, at the same time,they are a suitable matrix for the immobilizationand entrapment of enzymes (Yang et al. 2014).For instance, Yang et al. compared theperformance of enzyme-entrapped conductingpolymer nanofibers (NFs) with reference to itsconducting polymer film (F) counterpart. Amongdifferent conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has beenchosen as support for GOD enzyme because of its

chemical stability and good electricalconductivity. Poly(L-lactide) (PLLA) has beenelectrospun on Pt microelectrodes followed bythe electrochemical deposition of PEDOT-GODon the Pt microelectrodes and around PLLAnanofibers. Comparing the performance of thetwo different nanostructured sensors (PEDOT-Fvs. PEDOT-NFs), it has been shown that theincrease of surface area related to the nanofibersaffects both the impedance of the biosensor(which is sensitively lower) (Abidian et al. 2010)and it allows more GOD to be entrapped on thebiosensor. Both of these results contribute to theincreased sensitivity of the PEDOT-NFs-GODbiosensors.

Nevertheless, the scientific research on biosen-sors is moving also toward the design and devel-opment of nonenzymatic glucose sensors basedon direct oxidation of glucose at modified elec-trode surface (Kong et al. 2012). According toDing et al. (2010), metals (Au, Pt, Ni, Cu) andmetal oxides (ZnO, CuO, etc.) have beenexploited to construct a variety of enzyme-freeglucose sensors. Among the aforementionedmetal oxides, cobalt oxide nanofibers (Dinget al. 2010) have been obtained by electrospinningand subsequent calcination, and they have beenused for glucose detection in an alkaline medium.The developed sensor has been shown to have afast response time (less than 7 s), a high sensitivityof 36.25 mA mM�1cm�2, good reproducibilityand selectivity, and a detection limit of 0.97 mM(S/N = 3). The high sensitivity and low detectionlimit has been hypothesized to be related to theexcellent catalytic property of the as-preparedCo3O4 nanofibers and to their highly porous 3Dnetwork. The latter provides high specific surfacearea and numerous active sites, and it allows theaccess of analytes to the active catalytic sites withminimal diffusion resistance. Another attempt toget nonenzymatic glucose sensor has beenreported in the research of Cao et al. (2011),wherein nickel oxide microfibers (NiO-MFs)have been directly immobilized onto the surfaceof fluorine tin oxide (FTO) electrode byelectrospinning followed by calcinationwithout using any immobilization matrix. Theamperometric performance of glucose at

662 Electrospun Nanofiber Membrane for Biosensors

NiO-MFs-modified electrode have shown anultrasensitive current (1785.41 mA mM�1 cm�2)response and low detection limit of 3.3 � 10�8M(S/N = 3).

Despite the presence of enzyme-based andnonenzyme-based biosensors, there are also othersupport-enzyme hybrid substrate materials for bio-sensors. An example that has been investigated byresearchers was based on water-stable PVA andPVA/poly(ethyleneimine) (PEI) nanofibers deco-rated with silver nanoparticles (AgNPs) (Zhuet al. 2013). By combining an in situ reductionapproach and electrospinning technique, a H2O2,glutathione (GSH), and glucose detectionbiosensor has been obtained, showing a high sen-sitive detection of H2O2 with a detection limit of5 mM and a fast response, broad linear range, lowdetection limit, and excellent stability andreusability.

Looking at the aforementioned literature, itappears evident that the ongoing trend in biosen-sor fabrication is the use of specific nanomaterialsto get better sensitivity, lower detection limit,and linear response range. Further evidence ofthis is the H2O2-modified electrode based onhemoglobin (Hb) collagen and carbon nanotubes(CNTs) proposed by Li et al. (2014). Hb has beenused because of its similar structure to peroxidase,CNTs for the electrical properties, and collagenfor its excellent biocompatibility. Therefore,the resulting hemoglobin-collagen-carbon nano-tube (Hb-collagen-CNT) composite nanofiber-modified electrode has excellent performance,thanks to the great surface area and thehigh porosity of the three-dimensional reticularstructure which ensures more channels for bothelectron transfer and diffusion of H2O2.

Among enzymes currently used in biosensorfabrication, urease (Sawicka et al. 2005), fructosedehydrogenase (Marx et al. 2011),a-chymotrypsin (a proteolytic enzyme acting inthe digestive systems of many organisms) (Jiaet al. 2002), and lipase (Li et al. 2007; Yeet al. 2006) represent just a few examples of theongoing research. The large surface area ofelectrospun nanofibers has been found to beimportant for increasing both the amount ofimmobilized enzymes and their own catalyzing

activity, thanks to an effective reduction of thediffusion resistance of the substrates/products.

Further challenge in detecting DNA repair rep-resents a key factor in cancer diagnostic. Due tothe subtlety of DNA damage, it is difficult to sensethe presence of damaged repair with high selec-tivity and sensitivity. In this regard immobilizingDNA on gold/polymeric electrospun nanofibersand their use in electrochemical sensing of DNArepair processes have been shown to be very prom-ising, showing the potential of these low-costdevices for sensing applications (McWilliamset al. 2014). Polyacrylonitrile electrospun fiberscoated with gold nanoparticles have been testedand thiolated DNA has been assembled on thesefibers. It has been demonstrated that the fibersthemselves become the working electrode of thesensors, modified with either the lesion-bearing ordefect-free DNA monolayers. From the reportedassays, the device sensitivity has been found to beon the order of femtomoles per electrode, and itmeans that sensing can be accomplished with even1 ng of enzyme.

References

Abidian MR, Corey JM, Kipke DR, Martin DC(2010) Conducting-polymer nanotubes improve electri-cal properties, mechanical adhesion, neural attachment,and neurite outgrowth of neural electrodes. Small 6:421

Bhardwaj N, Kundu SC (2010) Electrospinning: a fasci-nating fiber fabrication technique. Biotechnol Adv28:325–347

Cao F, Guo S,MaH, Shan D, Yang S, Gong J (2011) Nickeloxide microfibers immobilized onto electrode byelectrospinning and calcination for nonenzymatic glu-cose sensor and effect of calcination temperature on theperformance. Biosens Bioelectron 26:2756–2760

D’Orazio P (2003) Biosensors in clinical chemistry. ClinChim Acta 334:41–69

Ding Y, Wang Y, Su L, Bellagamba M, Zhang H, Lei Y(2010) Electrospun Co3O4 nanofibers for sensitive andselective glucose detection. Biosens Bioelectron26:542–548

Jia HF, Zhu GY, Vugrinovich B, Kataphinan W, RenekerDH, Wang P (2002) Enzyme-carrying polymericnanofibers prepared via electrospinning for use asunique biocatalysts. Biotechnol Prog 18:1027–1032

Kong FY, Li XR, Zhao WW, Xu JJ, Chen HY(2012) Graphene oxide–thionine–Au nanostructure com-posites: preparation and applications in non-enzymaticglucose sensing. Electrochem Commun 14:59–62

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Li SF, Chen JP, Wu WT (2007) Electrospun polyacryloni-trile nanofibrous membranes for lipase immobilization.J Mol Catal B: Enzym 47:117–124

Li J, Mei H, Zheng W, Pan P, Sun XJ, Li F, Guo F, ZhouHM, Ma JY, Xu XX, Zheng YF (2014) A novel hydro-gen peroxide biosensor based on hemoglobin-collagen-CNTs composite nanofibers. Coll Surf 284B. 118:77–82

Marx S, Jose MV, Andersen JD, Russel AJ(2011) Electrospun gold nanofiber electrodes for bio-sensors. Biosens Bioelectron 26:2981–2986

McWilliams MA, Anka FH, Balkus JK, Slinker JD(2014) Sensitive and selective real-time electrochemi-cal monitoring of DNA repair. Biosens Bioelectron54:541–546

Ramakrishna S, Fujihara K, Teo WE, Lim TC, Ma Z(2005) An introduction to electrospinning andnanofibers. World Scientific, Singapore

Ren G, Xu X, Liu Q, Cheng J, Yuan X, Wu L, Wan Y(2006) Electrospun poly(vinyl alcohol)/glucose oxi-dase biocomposite membranes for biosensor applica-tions. React Funct Polym 66:1559–1564

Rodriguez-Mozaz S, Marco MP, Lopez de Alda MJ,Barcelò D (2004) Biosensors for environmental moni-toring of endocrine disruptors: a review article. AnalBioanal Chem 378:588–598

Sawicka K, Gouma P, Simon S (2005) Electrospunbiocomposite nanofibers for urea biosensing. SensorActuat B-Chem 108:585–588

Thevenot DR, Toth K, Durst RA, Wilson GS (2001) Elec-trochemical biosensors: recommended definitions andclassification. Biosens Bioelectron 16:121–131

Wendorff JH, Agarwal S, Greiner A (2012)Electrospinning. Materials, processing and applica-tions. Wiley-VCH, Weihneim

Wilson GS, Gifford R (2005) Biosensors for real-time invivo measurements. Biosens Bioelectron 20:2388

Wu J, Fan Y (2013) Sensitive enzymatic glucose biosensorfabricated by electrospinning composite nanofibers andelectrodepositing Prussian blue film. J ElectroanalChem 694:1–5

Wu J, Yin F (2013) Sensitive enzymatic glucose biosensorfabricated by electrospinning composite nanofibers andelectrodepositing Prussian blue film. J ElectroanalChem 694:1–5

Yang G, Kampstra KL, Abidian MR (2014) High-performance conducting polymer nanofiber biosensorsfor detection of biomolecules. AdvMater. doi:10.1002/adma.201400753

Ye P, Xu ZK, Wu J, Innocent C, Seta P (2006) Nanofibrousmembranes containing reactive groups: electrospinningfrom poly(acrylonitrile-co-maleic acid) for lipaseimmobilization. Macromolecules 39:1041–1045

Yoo EH, Lee SY (2010) Glucose biosensors: an overviewof use in clinical practice. Sensors 10:4558–4576

Zhu H, Du ML, Zhang Z, Wang P, Bao SY, Wang LN, FuYQ, Yao JM (2013) Facile fabrication of AgNPs/(PVA/PEI) nanofibers: high electrochemical efficiencyand durability for biosensors. Biosens Bioelectron49:210–215

Electrospun Nanofibers for Waterand Wastewater TreatmentApplications


Water pollution is the contamination of waterbodies (e.g., lakes, rivers, oceans, aquifers, andgroundwater), and it occurs when pollutants aredischarged directly or indirectly into water bodieswithout adequate treatment to remove harmfulcompounds. Direct sources include effluent out-falls from factories, refineries, waste treatmentplants, etc., that emit fluids of varying qualitydirectly into urban water supplies. Indirectsources include contaminants that enter the watersupply from soils/groundwater systems and fromthe atmosphere via rainwater. Soils and ground-waters contain the residue of human agriculturalpractices (fertilizers, pesticides, etc.) and improp-erly disposed of industrial wastes. Atmosphericcontaminants are also derived from human prac-tices (such as gaseous emissions from automo-biles, factories, and even bakeries).Contaminants can be broadly classified intoorganic, inorganic, radioactive, and acid/base.Moreover, bacteria and virus represent a danger-ous class of contaminating agents, which areresponsible of diseases in developing countries.

Microbial Control of Water

Currently the basic water treatments for microbialcontrol require chemical disinfectants andmembrane-based filtration systems. If the forma-tion of harmful disinfection by-products fromchemical disinfectants is the potential drawbackthat makes their use not completely reliable(Li et al. 2008), water filtration membrane isaffected by biofouling and virus penetration. Bio-fouling can be considered as a biotic form of

664 Electrospun Nanofibers for Water and Wastewater Treatment Applications

organic fouling, and it represents a major problemin nanofiltration (NF) and reverse osmosis(RO) membrane filtration.

At this regard, electrospun nanofibers havebeen found to be a good candidate for improvingthe performance of filtering media. Antifoulingproperties can be achieved by plasma treatmentor surface graft polymerization of nanofibers, assuggested by the literature (Musale and Kumar2000). Another advantage related to the use ofelectrospun nanofibers for microbial control isthe capability to get a wide range of assemblies,for instance, antimicrobial nanofibers themselvesand nanobiocides, which include metal, metal-oxide nanoparticles, engineered nanomaterials(fullerenes and carbon nanotubes), and naturalsubstances (Botes and Cloete 2010). For instance,nanoparticles of Ag, TiO2, and ZnO representlow-cost materials with a well-known antimicro-bial activity that can be properly attached tonanofibers reducing potential toxicity andleaching effects.

Polyurethane, poly(vinylidene) fluoride-co-hexafluoro-propylene (PVDF-HFP), and polycar-bonate with quaternary ammonium salt are justfew examples of antimicrobial nanofilters whichhave been shown to have a very good filtrationefficiency of 0.3mm size particles (99.93%) and avery strong antimicrobial activities against bothS. aureus and E. coli (Yao et al. 2008, 2009; Kimet al. 2007).

Desalination

Another challenging task related to the water issueregards the need of alternative sources in order totackle the higher demand for fresh water andenergy resources. Desalination technology thatconverts seawater into clean water has beenfound to be a suitable approach for it. Amongthe different membrane separation processes,membrane distillation (MD), electrodialysis(ED), reverse osmosis (RO), freeze desalination(FD), ion exchange (IX), and nanofiltration(NF) have been developed as desalination tech-nologies (Sundarrajan and Ramakrishna 2013).

Nanofiltration is a type of pressure-driven mem-brane process with properties in between reverseosmosis (RO) and ultrafiltration (UF), which hasbeen found to be adequate for desalination withlow salt content. According to the establishedtechnology, in order to reduce the resistance andenhance the flux, thin film composite membranes(TFC) are commonly used. They are based onthree layer, that is, a nonwoven fabric as support,a middle porous support, and a top layer which isresponsible for the selective salt removal. Withinthis scenario, electrospun nanofibers can be usedas a mid-layer membrane in order to achievehigher flux and low fouling behavior than thecommercial NF membranes (Yung et al. 2010).Nevertheless, the applicability of electrospunmembranes as self-supporting NF membraneshas been proved (Kaur et al. 2012) even thougha careful optimization of the whole system needsfurther studies in terms of process optimizationand cost analysis.

As regards desalination by membrane distilla-tion (MD), nanofibrous membranes based onelectrospun polymers (polyvinylidenefluoride,PVDF) and copolymers (PVDF-co-F6PP, PVDF-co-hexafluoropropene) have been proven toreduce the energy requirements of direct contactmembrane distillation process, thanks to thehigher permeate flows (Khayat et al. 2011; Shih2011) than PTFE commercial membranes. Verygood performance in a direct contact membranedistillation setup could be achieved also by self-supporting carbon nanotube (CNT) membranes(Dumée et al. 2011), which have to be properlymodified for enhancing their hydrophobicity andthat allow to get a 99 % salt rejection and a fluxrate of 12 kg/m2.

Heavy Metal Ions Removal

Water pollution by heavy metal ions is anotherimportant related issue because long-term expo-sure to metal ions such as lead (Pb) can result inacute or chronic damage to the brain and thecentral nervous system on humans. Furthermore,heavy metals are dangerous because they tend to

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bioaccumulate, i.e., the concentration of a chem-ical increases in a biological organism over time,compared to the chemical’s concentration in theenvironment (Lee et al. 2013). Among the tech-nologies employed for the treatment of contami-nated water (chemical oxidation, precipitation, IE,RO), adsorption and membrane separation couldbe enhanced by electrospun nanofibers. In theliterature several examples are reported, the cen-tral ideas of which are:

i. The functionalization of the fibers with suchmolecules able to bind metal ions

ii. The surface modification of polymer in orderto vary certain properties, such as hydrophi-licity or hydrophobicity, water absorbency,adsorption capacity, and resistance to micro-bial attack

iii. The production of mixed nanocompositefibers

As regards the functionalization of fibers (pointi), rhodanine is a heterocyclic molecule thatbelongs to the sulfur-containing N and O organiccompounds which possesses uptake performancetoward Ag (I), Pb (II), and Hg (II) ions (Leeet al. 2013). On the other hand cellulose andchitosan-/polyacrylamide-modified nanofibershave been proven to be suitable for the adsorptionof trace of metals (Cd, Pb, Cu, Cr, and Ni) fromthe river water and treated water (Musyokaet al. 2013). Their adsorption capacity has beenenhanced (point ii) by surface modificationusing furan-2,5-dione. The electrospun ironoxide–alumina fiber systems represent an exam-ple of nanocomposite fibers (point iii) that havebeen used as adsorbent for the removal of heavymetal ions, i.e., Cu2+, Ni2+, Pb2+, and Hg2+ fromaqueous system (Mahapatra et al. 2013).

References

Botes M, Cloete TE (2010) The potential of nanofibers andnanobiocides in water purification. Crit Rev Microbiol36:68–81

Dumée L, Germain V, Sears K, Sch€utz J, Finn N, Duke M,Cerneaux S, Cornu D, Gray S (2011) Enhanced

durability and hydrophobicity of carbon nanotubebucky paper membranes in membrane distillation.J Membr Sci 376:241–246

Kaur S, Sundarrajan S, Gopal R, Ramakrishna S(2012) Formation and characterization ofpolyamide composite electrospun nanofibrous mem-branes for salt separation. J Appl Polym Sci 124:E205–E215

Khayet M, Payo G, Carmen AS, Carmen M (2011) Nano-structured flat membranes for direct contact membranedistillation. WO/2011/117443

Kim SJ, Nam YS, Rhee DM, Park HS, Park WH(2007) Preparation and characterization of antimicro-bial polycarbonate nanofibrous membrane. Eur PolymJ 43:3146–3152

Lee CH, Chiang CL, Liu SJ (2013) Sep Purif Technol118:737–743

Li Q, Mahendra S, Lyon DY, Brunet L, Liga MV, Li D,Alvarez PJJ (2008) Antimicrobial nanomaterialsfor water disinfection and microbial control:potential applications and implications. Water Res42:4591–4602

Mahapatra A, Mishra BG, Hota G (2013) ElectrospunFe2O3–Al2O3 nanocomposite fibers as efficientadsorbent for removal of heavy metal ionsfrom aqueous solution. J Hazard Mater 258–259:116–123

Musale DA, Kumar A (2000) Solvent and pH resistance ofsurface crosslinked chitosan/poly(acrylonitrile) com-posite nanofiltration membranes. J Appl Polym Sci77:1782–1793

Musyoka SM, Ngil JC, Mamba BB (2013) Remediationstudies of trace metals in natural and treated water usingsurface modified biopolymer nanofibers. Phys ChemEarth 66:45–50

Shih JH (2011) A study of composite nanofiber membraneapplied in seawater desalination by membrane distilla-tion. Master’s Thesis, National Taiwan University ofScience and Technology

Sundarrajan S, Ramakrishna S (2013) New directions innanofiltration applications – are nanofibers the rightmaterials as membranes in desalination? Desalination308:198–208

Yao C, Li X, Neoh KG, Shi Z, Kang ET(2008) Surface modification and antibacterial activityof electrospun polyurethane fibrous membranes withquaternary ammonium moieties. J Membr Sci320:259–267

Yao C, Li X, Neoh KG, Shi Z, Kang ET (2009)Antibacterial activities of surface modified electrospunpoly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP) fibrous membranes. Appl Surf Sci255:3854–3858

Yung L, Ma H, Wang X, Yoon K, Wang R, Hsiao BS,Chu B (2010) Fabrication of thin-film nanofibrouscomposite membranes by interfacial polymerizationusing ionic liquids as additives. J Membr Sci365:52–58, new container

666 Electrospun Nanofibrous Membranes

Electrospun NanofibrousMembranes

Martina Roso, Carlo Boaretti, AlessandraLorenzetti and Michele ModestiDepartment of Industrial Engineering, Universityof Padova, Padova, Italy

Introduction

Nanofibers are an interesting and versatile class ofone-dimensional (1-D) nanomaterials, with diam-eters ranging from tenths to hundreds of nanome-ters, which have been recognized as promisingdue to their outstanding properties in terms ofhigh porosity, excellent pore interconnectivity,small diameters, and high surface-to-volumeratio. Among the different nanofiber manufactur-ing technologies, electrostatic spinning orelectrospinning represents the easiest, most prom-ising, and versatile method for the generation ofaligned or randomly distributed nanofibers of arich variety of different materials like syntheticand natural polymers (Huang et al. 2003), com-posites (Sahay et al. 2012), ceramics (Daiet al. 2011), and metals (Wu et al. 2007).

With this technique that is essentially a varia-tion of the electrostatic spray (or electrospraying),a fiber is produced thanks to the solidification ofan electrified jet of a solution that is stretched bythe repulsive action of surface charges and evap-oration of solvent. The main difference withrespect to electrospraying is the presence of highelongational viscous forces generated by the chainentanglements of the polymer present in the solu-tion that, thanks to the application of a strongelectric field, prevent the formation of liquid drop-lets in favor of a rapid whipping jet. Theelectrospun solution is essentially polymerbased, with the possibility to add metal or ceramicprecursors that after electrospinning treatmentscan generate the corresponding nanofibers.

An extensive research activity has been carriedin the last years toward the evaluation ofelectrospun nanofibrous membranes (ENMs) inthe context of membrane technology, fueled by

the advantages offered in terms of cost and pro-ductivity over more complicated bottom-upapproaches, flexibility of the membranes, andthe potential upscaling for industrial production(Persano et al. 2013).

Fundamentals

The generation of nanofibers by electrospinningstarts from a solution of a proper solvent in whichthe desired material is dissolved at a suitable con-centration. This solution is usually poured in asyringe or a pipette from which it is let flownthrough a nozzle using a micropump, and bymeans of a high-voltage direct current power sup-ply (5–50 kV), an electrostatic field is appliedbetween the nozzle and a metal collector withopposite polarity. By increasing the intensity ofthe electric field, a charge is induced on the sur-face of the solution at the tip of the capillary. Themutual repulsion of the charges elongates thehemispherical drop to form a conical shape(Taylor cone). Once that the electric field hasreached a threshold value, the electrostaticforce overcomes the surface tension and acharged jet of solution is ejected from the tip ofthe Taylor cone, producing a jet that travelstoward the metal collector. During this last phasethe jet undergoes a whipping process in which it isstretched and the solvent of the solution evapo-rates leaving a fiber that is collected on a groundedcollector.

The morphology and diameter of electrospunnanofibers depend on a wide range of parameters,which can be classified according to three maindifferent categories:

– Solution parameters: type of solvent, viscosity,molecular weight and molecular weight distri-bution, vapor pressure, surface tension, andsolution conductivity

– Process parameters: solution feeding rate,electric field strength, distance between thetip and the collector, needle diameter, compo-sition, geometry, and motion of the collector

– Ambient parameters: temperature, humidity,and air velocity.

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Between them viscosity has a prominent role inrelation to the morphology of the nanofibers. Forpolymer solutions of low viscosity, surface ten-sion is the dominant aspect and just beads orbeaded nanofibers are obtained. As the viscosityof the solution is increased, the beads becomebigger, the average distance between them longer,and the shape of the beads changes from sphericalto spindle-like with a decrease of the fiber diam-eter. At a suitable concentration, smoothnanofibers without defects can be obtained, butwith a progressive increment of its value, thefibers increase their diameter until they changetheir shape into micro-ribbons. Usually the properviscosity is defined in relation to the specificpolymer-solvent system, and for a givenpolymer-solvent system, it can change accordingto the molecular weight of the polymer. Complexarchitectures like core-shell, porous, and hollowstructures can be formed by electrospinningthrough the incorporation of nanoparticles intofibers, co-electrospinning of different polymers,or employing solutions with different boilingpoint solvents, while modification of theelectrode-collector system allows to orientate thefibers and to obtain 3-D structures (Teoet al. 2011).

Electrospun nanofibers can also be obtainedfrom polymer melts at high temperature, from acombination of a magnetic and electric field, aer-ated polymer solution, and from multi-jet devicesin order to improve production rate.

Applications

The interesting properties related to the possibilityof obtaining nanometric fibers fromelectrospinning allow their use for several poten-tial applications, with a multidisciplinary perspec-tive. Broadly speaking it is possible to identifyfour main areas of interest (Ramakrishnaet al. 2005): bioengineering, environmental engi-neering and biotechnologies, energy and electron-ics, and, finally, defense and security. In thisframework the specific fields of applications inwhich electrospun nanofibers are directly in con-tact with membrane technology are several,

including the most traditional and advanced tech-nologies: air and water filtration, fuel cell mem-branes and battery membrane separators,biomedical applications, and many others. Mostof these applications haven’t already reached theirindustrial level, but the promising results from theresearch field and their different potential applica-tions are believed to encounter the interest ofindustry and governments. Actually the mostindustrially developed products based on ENMsare filters for air purification. Potential productsready to market are related to liquid filtration andenergy storage, in the short-medium period, and tothe biomedical field, in the medium-long period(Persano et al. 2013).

Air and Water FiltrationENMs’ high specific surface, porosity and perme-ability, tailorable thickness, and fiber diametersare desirable features for filtration applications.These materials have been employed for severalyears for air filtration and recently, at a researchlevel, even for microfiltration, ultrafiltration, andnanofiltration in the field of water treatment. Theadvantages of electrospun nanofibers compared toconventional air filter media are related to a higherfiltration efficiency, lower pressure drops, andenergy savings. Indeed, nanofibers with diameterslower than 0.5 mm have much higher capability tocollect fine particles because of the slip flow thatincreases diffusion, interception, and inertialimpaction efficiencies, determining a lower dragforce around the fibers and, thus, lower pressuredrops. In addition these media are easy to clean,enabling them to significantly extend the life offilters and reducing the maintenance costs. Theirpotentials have shown possible applications in airfiltering media (Barhate and Ramakrishna 2007),especially for high-efficiency particulate andlow-penetration air filters, filters for transportationapplications, adsorptive catalytic gas filter for res-pirators, filter media for pulse clean cartridges indust collection, and cigarette filters for smokefiltration.

ENMs can be employed also for water treat-ment (Cloete et al. 2013) and generally show highflux rate and low transmembrane pressure withperformance comparable to those commercially

668 Electrospun Nanofibrous Membranes

available. The main interesting application in thisarea is related to desalination, VOC gas stripping,oil/water emulsion separation, microbial, organiccompounds, and heavy metal removal. However,for solid-liquid filtration, very high surface-to-volume ratios promote membrane fouling. Forthis reason and for improving membrane perfor-mances, ENMs can be surface modified (Nasreenet al. 2013), in order to add functional groups orfunctionality, with processes like in situ graft andinterfacial polymerization, plasma treatment, wettreatment, coating, and blending with surface-modifying agents. A main drawback for theemployment of electrospun nanofibers for suchapplications is their mechanical strength which isnot sufficient to withstand macroscopic impactduring filtration application such as normal liquidor air flows passing through them. Hence, theyneed to be used as active coating layer on existingmelt-blown supportive fibrous media (compositemembrane) or the fibers need to be bonded toenhance mechanical properties.

Fuel Cell and Battery Separator MembranesAnother area in which ENMs can have promisingapplications is that of energy-related applicationsand devices (Cavaliere et al. 2011), especially forpolymer fuel cell and lithium-ion battery electro-lyte membranes.

Research efforts on polymeric protonexchange membranes for fuel cells have led theinterest of researchers toward the employment ofENMs as porous reinforcing mats to minimizein-plane swelling and shrinking. Nanofibers arealso able to increase proton conductivity withrespect to bulk film, thanks to their highly orientedionic domains (Dong et al. 2010) and providegood mechanical strength, while the remarkableflexibility of their production process allows toadequately tailor their final morphology for com-posite membranes. Fuel cell membranes are semi-permeable membranes that have the function to beproton conductive, electron insulating, and densein order to avoid fuel crossover. For this reasontwo types of approaches can be used whenemploying electrospinning for such application.The first is related to the electrospinning of

nonconductive or less conductive polymer into aporous matrix, which acts as mechanical rein-forcement when the pores are filled with a highlyproton-conductive component. Alternatively, ahighly proton-conducting matrix is electrospuninto a porous fiber mat and subsequentlyreinforced with a secondary polymer to providemechanical stability. The proton-conductive poly-mers are usually chosen between perfluoro-sulfonic acid and sulfonated polymers that canbe coupled with organic or inorganic particles asconductor enhancers or to improve strength andhydrophilicity.

Electrospun mats are also attractive alterna-tives to polymer gel electrolytes for lithium-ionbatteries, since they can be employed as matrix inwhich the electrolyte can be encapsulated improv-ing ionic conduction across the membrane andmechanical strength while providing good wateruptake. In these fields the most widely studiedpolymer has been poly(vinylidene fluoride)(PVDF) thanks to its good electrochemical andthermal stability. However, PVDF-based gel poly-mer electrolyte with its high crystallinity limits theion migration, lowering the battery performance.ENMs with their porous membrane are capable toovercome this problem, and PVDF and otheralternative polymers and blend have beenexplored by electrospinning with encouragingresults.

Biomedical ApplicationsNanofiber research applications in the biomedicalfield have a multifaceted perspective spreadingbetween tissue engineering, drug delivery, andwound dressing (Agarwal et al. 2008). Tissueengineering is an emerging multidisciplinaryarea in which nanofibers represent an importantadvancing front for the production of suitablescaffolds of different materials that can mimicnatural extracellular matrix. To this purposeENMs have been tested as natural, synthetic, andcomposite scaffolds for different types of targetedtissue including blood vessel, cartilage, bone,nerve, and many others. The small diameter ofthe fibers and their high surface area are beneficialfor cell attachment and bioactive factor loading,enhancing cell growth.

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Drug delivery membranes incorporate a drugcomponent that can be patched on wound of sur-gery or encapsulated into pharmaceutical capsulesto deliver the drug through the digestive system ofthe patient. Polymeric electrospun drug deliverysystems are advantageous for such task becausethey can deliver drugs efficiently to a localizedarea, with the possibility to vary the release rate bysimply varying the fiber diameter or the loadingdosage. The possibility of choosing differentmaterials, processes, and processing conditionsallows to reach the desired encapsulation effi-ciency, preserving bioactivity (Zamaniet al. 2013). Several drugs have been incorporatedsuccessfully into electrospun media obtaining bet-ter performance over normal cast film and with thepossibility to load insoluble drug for enhancingtheir dissolution.

ENMs have also exhibited potential in wounddressing thanks to the possibility to generatehomogeneous scaffolds, provide uniformadherence, and wet wound surface without fluidaccumulation. They can provide high gas perme-ation and protection from infection and dehydra-tion, extending their applications on differenttypes of wounds as compared to traditionalwound dressing materials and opening newdoors for the next generation of wound dressingmaterials.

Other ApplicationsElectrospun materials from stable polymers orceramic fibers are also ideal candidates as sup-ports for homogeneous and heterogeneous catal-ysis in gas phase, since they can provide highsurface area and high porosity. The catalyst,which is usually a semiconductor, can be thenanofibrous mat itself, or it can be subsequentlyadded by surface modifications or deposition.Examples are organic or inorganic nanofibers onwhich metals or semiconductor nanoparticles aredeposited or embedded for catalytic (Basheer2013) and photocatalytic oxidation (Modestiet al. 2014) and synthesis (Formo et al. 2009) oforganic compounds in gas phase. The advantagesof the high surface areas of the catalytic filtersobtainable with this technique are related tohigher activity with respect to common porous

catalysts for the abatement of selectedcompounds.

The characteristics of ENMs match well therequirements for different types of sensingdevices (Ding et al. 2009) including acousticwave, resistive, gravimetric, photoelectric, opti-cal, and amperometric sensors. Indeed, ENMs’high surface area has the potential to provideunusually high sensitivity, fast response time,and lower detecting limits. In this case differentapproaches can be employed to provide a sensingcapability to nanofibers, such as electrospinningof a polymeric sensing material, incorporation ofsensing molecules into nanofibers, or applicationof sensing material on nanofiber surface via coat-ing/grafting technique, employing organic andinorganic polymer.

Other potential applications of ENMs includeaffinity membranes for protein purification(Ma et al. 2005) and protective clothing againstnanoparticles (Faccini et al. 2012) on textilesupport.

Cross-References

▶Nanofiber▶Oil-Water Emulsion▶ Proton-Exchange Membranes for Fuel Cells▶Ultrafiltration (UF)

References

Agarwal S, Wendorff JH, Greiner A (2008) Use ofelectrospinning for biomedical applications. Polymer49:5603–5621

Barhate RS, Ramakrishna S (2007) Nanofibrous filteringmedia: filtration problems and solutions from tinymaterials. J Membr Sci 296:1–8

Basheer C (2013) Nanofiber-membrane-supported TiO2 asa catalyst for oxidation of benzene to phenol. J Chem2013:1–7

Cavaliere S, Subianto S, Savych I, Jones DJ, Roziere J(2011) Electrospinning: designed architectures forenergy conversion and storage devices. Energy Envi-ron Sci 4:4761–4785

Cloete TE, de Kwaadsteniet M, Gule NP, Klumperman B(2013) Electrospun nanofibrous membranes for watertreatment applications. In: Lens PNL, Virkutyte J,Jegatheesan V, Kim SH, Al-Abed S (eds)

http://dx.doi.org/10.1007/978-3-662-44324-8_1538

http://dx.doi.org/10.1007/978-3-662-44324-8_730

http://dx.doi.org/10.1007/978-3-662-44324-8_1309

http://dx.doi.org/10.1007/978-3-662-44324-8_1206

Polyethersulfone (PES)

O

O n

SO

Electrospun Polyethersulfone Nanofiber Mem-branes, Fig. 1 Repetitive unit of polyethersulfone

670 Electrospun Polyethersulfone Nanofiber Membranes

Nanotechnology for water and wastewater treatment.IWA Publishing, London, pp 283–294

Dai Y, Liu W, Formo E, Sun Y, Xia Y (2011) Ceramicnanofibers fabricated by electrospinning and theirapplications in catalysis, environmental science,and energy technology. Polym Adv Technol22:326–338

Ding B,WangM, Yu J, Sun G (2009) Gas sensors based onelectrospun nanofibers. Sensors 9:1609–1624

Dong B, Gwee L, Salas-de la Cruz D, Winey KI, Elabd YA(2010) Super proton conductive high-purity Nafionnanofibers. Nano Lett 10:3785–3790

Faccini M, Vaquero C, Amantia D (2012) Development ofprotective clothing against nanoparticle based onelectrospun nanofibers. J Nano Mater 2012(892894) 9pages. doi:10.1155/2012/892894

Formo E, Yavuz MS, Lee EP, Lane L, Xia Y (2009)Functionalization of electrospun ceramic nanofibremembranes with noble-metal nanostructures for cata-lytic applications. J Mater Chem 19:3878–3882

Huang ZM, Zhang YZ, Kotaki M, Ramakrishna S (2003)A review on polymer nanofibers by electrospinning andtheir applications in nanocomposites. Compos SciTechnol 63:2223–2253

Ma Z, Kotaki M, Ramakrishna S (2005) Electrospunnanofiber as affinity membrane. J Membr Sci265:115–123

Modesti M, Roso M, Boaretti C, Besco S, Hrelja D,Sgarbossa P, Lorenzetti A (2014) Preparation of smartnano-engineered electrospun membranes for methanolgas-phase photoxidation. Appl Catal B 144:216–222

Nasreen SAAN, Sundarrajan S, Nizar SAS,Balamurugan R, Ramakrishna S (2013) Advancementin electrospun nanofibrous membranes modificationand their application in water treatment. Membranes3:266–284

Persano L, Camposeo A, Tekmen C, Pisignano D (2013)Industrial upscaling of electrospinning and applicationsof polymer nanofibers: a review. Macromol Mater Eng298:504–520

Ramakrishna S, Fujihara K, Teo WE, Lim TC, MaZ (2005) An introduction to electrospinning andnanofibers. World Scientific, Singapore

Sahay R, Kumar PS, Sridhar R, Sundaramurthy J,Venugopal J, Mhaisalkar SG, Ramakrishna S(2012) Electrospun composite nanofibers and theirmultifaceted applications. J Mater Chem22:12953–12971

Teo WE, Inai R, Ramakrishna S (2011) Technologicaladvances in electrospinning of nanofibers. Sci TechnolAdv Mater 12:013002 (19pp)

Wu H, Zhang R, Liu X, Lin D, Pan W (2007)Electrospinning of Fe, Co, and Ni nanofibers: synthe-sis, assembly and magnetic properties. Chem Mater19:3506–3511

Zamani M, Prabhakaran M, Ramakrishna S (2013)Advances in drug delivery via electrospun andelectrosprayed nanomaterials. Int J Nanomedicine8:2997–3017

Further ReadingsAndrady AL (2008) Science and technology of polymer

nanofibers. Wiley, HobokenDing B, Yu J (2014) Electrospun nanofibers for energy and

environmental applications, Nanostructure science andtechnology series. Springer, Berlin

Jayakumar R, Shantikumar N (2012) Biomedical applica-tions of polymer nanofibers, advances in polymer sci-ence. Springer, Berlin

Modesti M, Lorenzetti A, Roso M (2011) Nanofibers viaelectrospinning. Encyclopedia of nanoscience andnanotechnology, vol 17. America Scientific, Valencia,California, pp 231–312

Wendorff JH, Agarwal S, Greiner A (2012)Electrospinning: materials, processing and applica-tions. Wiley-VCH, Singapore

Electrospun PolyethersulfoneNanofiber Membranes


Electrospun polyethersulfone (PES) nanofibermembranes are semipermeable membranesobtained by electrospinning that have beenexplored for possible applications concerningwater filtration (microfiltration, nanofiltration,and engineered osmosis), composite polymerelectrolyte membranes, and tissue engineering.

Polyethersulfone (PES) is a high-performancearomatic polymer which belongs to the family ofpolysulfones. The basic repeating unit of the polymerbackbone consists of para-linked aromatic groupsconnected by ether and sulfone groups (see Fig. 1).

PES is synthesized by polycondensation of4,4-sulfonylbisphenol (bisphenol S) with

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4,4’-dichlorophenyl sulfone via nucleophilic sub-stitution at high-reaction temperature (up to285 �C) in the presence of a suitable solvent likediphenyl sulfone, sulfolane, or N-methyl-2-pyrrolidone (Parker et al. 2012).

PES is a widely employed polymer for mem-brane fabrication due to its interesting properties interms of high glass transition temperatures, goodmechanical strength and stiffness, and outstandingthermal and oxidative resistance. This polymer canbe electrospunned from solutions of suitable con-centration with polar aprotic solvents (e.g.,dimethylacetamide) in order to produce nanofiberswith average diameters of 0.75–2 mm and averagespecific surface area of 15–30 m2/g, by changingthe process parameters (Kwak et al. 2013).

Electrospun nanofibrous membranes (ENMs)are a new class of energy-saving membranes thatare under extensive study because of their highinterconnected porosity and tunable pore size thatcan give high permeability and selectivity. One ofthe most important fields of interest for ENMs iswater filtration for which these membranes couldovercome some intrinsic limitations of conven-tional porous polymeric membranes. HoweverENMs suffer of poor mechanical strength, whilePES, even if extensively used for commercialmembranes, is a highly hydrophobic polymer sothe synthesis of composite membranes is the usualapproach to solve these issues. PES ENMs can beemployed for dye and heavy metal removal byblending with polymers that incorporate func-tional groups that have binding/chelating capabil-ity (Min et al. 2012; Wu et al. 2014). Themechanical properties of these membranes canbe improved by interfiber adhesion/junction(Yoon et al. 2009; Homaeigohar et al. 2012),while the hydrophilicity can be increased by oxi-dation treatment (Yoon et al. 2009).A combination of both improvements can beachieved by incorporation of hydrophilic inor-ganic particles which can also allow a higherthermal stability (Homaeigohar et al. 2011). PESENMs can be employed even as a middle layer inthin film composite polymeric membranes forengineered osmosis with the advantage to providehigher osmotic fluxes than their commercial coun-terparts (Bui et al. 2011).

Other potential applications concern polymericscaffolds as substrate for stem cell culture made ofpristine (Christopherson et al. 2009;Ardeshirylajimi et al. 2013) and modified (Chuaet al. 2006; Shabani 2009) PES, for which both thefiber diameter and the type of functionalizationcan have a prominent role in the infiltration, dif-ferentiation, and proliferation of the cells.

The high stability of the polymer and its chainflexibility due its structure make PES a suitablecandidate as non-fluorinated replacement for pro-ton exchange membranes, especially for directmethanol fuel cells. By sulfonation it is possibleto introduce proton conducting functional groupsalong the polymer chain, while by electrospinninga suitable nanofibrous web can be produced. Thisnonwoven structure can be filled with Nafion toobtain a compact membrane with higher electro-chemical performances with respect to Nafion112 and Nafion 117 dense membranes (Shabaniet al. 2010; Hasani-Sadrabadi et al. 2011).

Cross-References

▶Electrospun Nanofibrous Membranes▶ Proton-Exchange Membranes for Fuel Cells▶Ultrafiltration (UF)

References

Ardeshirylajimi A, Hosseinkhani S, Parivar K,Yaghmaie P, Soleimani M (2013) Nanofiber-basedpolyethersulfone scaffold and efficient differentiationof human induced pluripotent stem cells into osteoblas-tic lineage. Mol Biol Rep 40:4287–4294

Bui NN, Lind ML, Hoek EMV, McCutcheon JR(2011) Electrospun nanofiber supported thin film com-posite membranes for engineered osmosis. J MembrSci 385–386:10–19

Christopherson GT, Song H, Mao HQ (2009) The influ-ence of fiber diameter of electrospun substrates onneural stem cell differentiation and proliferation. Bio-materials 30:556–564

Chua KN, Chai C, Lee PC, Tang YN, Ramakrishna S,Leong KW, Mao HQ (2006) Surface-animatedelectrospun nanofibers enhance adhesion and expan-sion of human umbilical cord blood hematopoieticstem/progenitor cells. Biomaterials 27:6043–6051

Hasani-Sadrabadi MM, Shabani I, Soleimani M,Moaddel H (2011) Novel nanofiber-based triple-layer

http://dx.doi.org/10.1007/978-3-662-44324-8_1545

http://dx.doi.org/10.1007/978-3-662-44324-8_1309

http://dx.doi.org/10.1007/978-3-662-44324-8_1206

672 Embryonic Stem (ES) Cell

proton exchange membranes for fuel cell applications.J Power Sources 196:4599–4603

Homaeigohar SS, Mahdavi H, Elbahri M (2011) Extraor-dinarily water permeable sol-gel formednanocomposite nanofibrous membranes. J ColloidInterface Sci 366:51–56

Homaeigohar S, Koll J, Lilleodden ET, Elbahri M (2012)The solvent induced interfiber adhesion and its influenceon the mechanical and filtration properties ofpolyethersulfone electrospun nanofibrousmicrofiltrationmembranes. Sep Purif Technol 98:456–463

Kwak NS, Jung WH, Park HM, Hwang TS(2013) Electrospun polyethersulfone fibrous mats: sul-fonation, its characterization and solution-phase ammo-nium sorption behavior. Chem Eng J 215–216:375–382

Min M, Shen L, Hong G, Zhu M, Zhang Y, Wang X,Chen Y, Hsiao BS (2012) Micro-nano structure poly(ether sulfones)/poly(ethyleneimine) nanofibrous affin-ity membranes for adsorption of anionic dyes and heavymetal ions in aqueous solution. Chem Eng J 197:88–100

Parker D, Bussink J, Hendrik T, van De Grampel J,Wheately GW, Dorf EH, Ostlinning E, Reinking K,Schubert F, Junger O, Wagener R (2012) Polymer,high-temperature. In: Ullmann’s encyclopedia ofindustrial chemistry, 7th edn. Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim

Shabani I, Haddadi-Asl V, Seyedjafari E, Babaeijandaghi,Soleimani M (2009) Improved infiltration of stem cellson electrospun nanofibers. Biochem Biophys ResCommun 382:129–133

Shabani I, Hasani-Sadrabadi MM, Haddadi-Asl V,Soleimani M (2010) Nanofiber-based polyelectrolytesas novel membranes for fuel cell applications. J MembrSci 368:233–240

Wu JJ, Lee HW, You JH, Kau YC, Liu SJ (2014) Adsorp-tion of silver ions on polypyrrole embeddedelectrospun nanofibrous polyethersulfone membranes.J Colloid Interface Sci 420:145–151

Yoon K, Hsiao BS, Chu B (2009) Formation of functionalpolyethersulfone electrospun membrane for water puri-fication by mixed solvent and oxidation processes.Polymer 50:2893–2899

Embryonic Stem (ES) Cell

Loredana De BartoloInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Embryonic stem (ES) cell is a totipotent stem cellderived from an early stage embryo, which is calledblastocyst. This stage is reached 4–5 days after

fertilization and contains from 64 to several hun-dred cells organized in an outer shell, thetrophectoderm, and the inner cell mass (ICM).The ICM is the locus of pluripotent cells destinedto yield all the tissues of the developed organism.In the process of obtaining embryonic stem cells,the trophectoderm is removed by immunosurgery,and the ICM is disaggregated and plated on feedercells. Ethical issues surround the derivation ofhuman ES cells from in vitro fertilized blastocysts.

Embryonic stem cells have pluripotency andindefinite replication characteristics. These cellshave the capacity to give rise to differentiated prog-eny representative of all three embryonic germlayers (ectoderm, endoderm, and mesoderm). EScells are able to differentiate in all cell types differ-ently from adult stem cells that can produce only alimited number of cell types. It is possible to modu-late the differentiation of stem cells in a given phe-notype by using specific growth factors andmolecules, which trigger the differentiation process.

ES cells for their ability of propagating them-selves indefinitely represent a valuable tool forboth research and regenerative medicine. Theycan serve as an unlimited source of any cell typein the body; human ES cells could yield highlyeffective in vitro models for use in drug discoveryprograms and provide a renewable source of cellsfor use in transplantation therapy. Cell therapiesbased on the use of ES cells have been proposedfor tissue replacement after injury or disease.

Adult stem cells, isolatable from bone marrow,adipose tissue, tooth pulp, and many other loca-tions of the body, are capable of self-renewal andcan be readily expanded ex vivo for several pas-sages without losing their self-renewal capacity.Mesenchymal stem cells can differentiate intomultiple tissue-forming cell lineages such as oste-oblasts, chondrocytes, adipocytes, tenocytes, andmyocytes. Recent work on the differentiation ofbone marrow-derived mesenchymal stem cellsinto neuron-like cells is another demonstration ofconsiderable plasticity of adult mesenchymalstem cells. Stem cells are often a preferred cellsource for regeneration of multiple cell lineagetissues. The ability to expand stem cells is desiredto generate cells for tissue engineering in clinicaland pharmaceutical applications (Rahaman and

Emulsification 673

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Mao 2005). Conventional methods for expandingstem cells or progenitor cells comprised polysty-rene culture dishes and component of extracellularmatrix such as collagen. Alternatively biocompat-ible and biodegradable materials have been pro-posed to support cells and promote theirdifferentiation and proliferation toward the forma-tion of a tissue (Piscioneri et al. 2011).

Advances in cell-based therapies would nothave been possible without the innovative designand fabrication of several generations of biomate-rials. Novel biomaterials with distinct propertiesare necessary to accommodate the growth andinteractions of multiple cell lineages in compositetissue constructs (Griffith 2000). Membrane sys-tems provide the temporary structural frameworkfor tissue-forming cells to synthesize extracellularmatrices and other functional components in theintended shape and dimensions (De Bartolo andBader 2013). They can respond on the keydemands for utilizing cell-based therapies to engi-neer composite tissue constructs with a purposefulorientation toward anatomic structures that thetissue-engineered constructs are intended to regen-erate. Development of new intelligent biomaterialsin synergywith cell biology will advance stem cell-based clinical therapeutics. Engineered membraneshave the potential to mimic and control the physi-cal, chemical, and biological factors necessary forguiding stem cell differentiation (Pavlicaet al. 2009). They are currently being investigatedto act as scaffolds to guide and improve 3D tissueformation, substrates to enhance cell culturingtechniques, vehicles for cell delivery, and sourcesof immobilized and/or time-released factors. Theycan be applied to the regeneration of numeroustissue types, including the liver, pancreas, bone,cartilage, skin, and nerves, and are being used forthe in vitro realization of physiological models.

References

De Bartolo L, Bader A (2013) Biomaterials for stem celltherapy: state of art and vision for the future. CRCPress, Boca Raton

Griffith LG (2000) Polymeric biomaterials. Acta Mater48:263–277

Pavlica S, Piscioneri A, Peinemann F, Keller M,Milosevic J, Staeudte A, Heilmann A, Schulz-

Siegmund M, Laera S, Favia P, De Bartolo L,Bader A (2009) Rat embryonic liver cell expansionand differentiation on NH3 plasma-grafted PEEK-WC-PU membranes. Biomaterials 30:6514–6521

Piscioneri A, Campana C, Salerno S, Morelli S, Bader A,Giordano F, Drioli E, De Bartolo L (2011) Biodegrad-able and synthetic membranes for the expansion andfunctional differentiation of rat embryonic liver cells.Acta Biomater 7:171–179

Rahaman MN, Mao JJ (2005) Stem cell-based compositetissue constructs for regenerative medicine. BiotechnolBioeng 91(3):261–284

Emulsification

Emma PiacentiniInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Emulsification is a process by which one phase isbroken up, dispersed, and distributed in a secondimmiscible or partially miscible phase (Leal-Calderon et al. 2007). Many different emulsifica-tion methods can be identified, and they can bedistinguished in nonmechanical and mechanicalmethods. The nonmechanical methods include thedispersed phase precipitation and the phase inver-sion. Changes in the phase behavior of the sub-stances to be emulsified, promoted by variation oftemperature or composition or by mechanicalstress, are used to achieve the desirable state ofthe system. The mechanical methods of producingemulsions include the use of high-speed mixers,colloid mills, high-pressure valve homogenizers,ultrasonic homogenizers, microfluidization, andmembrane emulsification. Depending on the natureof the starting materials, emulsification can be dis-tinguished into two categories. The creation of anemulsion directly from two separate liquids isdefined as primary emulsification, whereas thereduction in size of the droplets in preformed emul-sion is defined as secondary emulsification (Fig. 1).

The physical processes that occur during emul-sification can be highlighted by considering thebehavior of two immiscible liquids in a containersuch as oil and water. Their thermodynamically

Oil

Water

PRIMARY EMULSIFICATION

SECONDARY EMULSIFICATION

Emulsification, Fig. 1 Emulsification

674 Emulsification

most stable state consists of a layer of oil on top ofa layer of water that allows to minimize the con-tact area between the two phases. To create andemulsion, it is necessary to supply energy in orderto disrupt and mix the oil and water which isusually achieved by mechanical agitation. Thedroplets formed are constantly moving aroundand frequently collide and coalesce with neigh-boring droplets. The presence of an emulsifierprevents the merging together of the dropletsafter they are formed. The emulsifier adsorbs tothe surface of the droplets during emulsificationand forms a protective membrane that preventsthe droplets from coming close enough togetherto coalesce. The rates of droplet disruption, drop-let coalescence, and emulsifier adsorption within aparticular homogenizer depend on the flow profilethat the fluids experience: (i) laminar flow whichis a regular, smooth, and well-defined flow withrelatively low flow rate; (ii) turbulent flow whichis an irregular, chaotic, and ill-defined flow withrelatively high flow rate characterized by the for-mation of eddies within the fluid; and (iii)cavitational flow which is an extremely complexflow because of the formation of small cavitiesthat implode and generate shock waves. The ten-dency for one flow regime is a consequence of thebalance of viscous and inertial forces acting on thefluid expressed by the Reynolds number:

Re ¼ inertial forces

viscous forces¼ L vrc

�c

where L is some characteristic length of the sys-tem, v is the average fluid flow velocity, rc is thedensity of the fluid, and �c is the viscosity of the

fluid. When the viscous forces generated within afluid dominate the inertial forces (low Re), theflow profile is laminar; when the inertial forcesdominate (high Re) in the flow profile, it is turbu-lent. The size of the droplets produced by ahomogenizer depends on a balance between thetwo opposing physical processes: droplet disrup-tion and droplet coalescence. The interfacialforces that tend to hold the droplets together andthe disruptive forces generated within the homog-enizer that tend to pull the droplets apart areinvolved in droplet disruption process. To deformand disrupt a droplet during homogenization, it isnecessary to apply an external force that is signif-icantly larger than the interfacial force. The inter-facial force is described by the Laplace equation:

DP ¼ 4gd

where g is the interfacial tension between the twoliquids, d is the droplet diameter, and DP is theLaplace pressure which acts across the interfacetoward the center of the droplet. The equationindicates that the pressure required to disrupt adroplet increases as the interfacial tensionincreases or as the droplet size decreases. For adroplet to be broken up during homogenization,the disruptive forces must exceed the interfacialforces and their duration must be longer than thetime required for droplet deformation. The rela-tive magnitude of disruptive and interfacial forcesis characterized by the Weber number (We):

We ¼ disruptive forces

interfacial forces

Emulsification, Table 1 The type of homogenizers used for emulsification

Homogenizertype Droplet formation mechanism Productivity

Droplet size and sizedistribution

Energy density(J m�3)

High-speed mixer Droplets break up in TI, TV, andLV flow regime

Batch orcontinuous

>2 mm, polydisperse Low-high

Colloid mill Droplets break up in LV and TVflow regime

Continuous >1 mm, polydisperse Low-high103–108

High-pressurehomogenizer

Droplets break up in TI, TV, LV,and CI flow regime

Batch orcontinuous

>0.1 mm,polydisperse

Medium-high106–108

Ultrasonic Droplets break up in CI flowregime

Continuous >0.1 mm,polydisperse

Medium-high106–108

Microfluidization Droplets break up in TI and TVflow regime

Batch orcontinuous

<0.1 mm,polydisperse

Medium-high106 to 2 � 108

Membrane Droplet detachment by wall shearstress

Continuous >0.3 mm, narrow Low-medium<103–108

TI turbulent inertial, TV turbulent viscous, LV laminar viscous, CI cavitational

Emulsification by Membrane Operations 675

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During homogenization droplet-droplet collisionsare particularly rapid because of the intensemechanical agitation of the emulsion. Dropletcoalescence will depend on the time taken forthe emulsifier to be adsorbed to the surface ofthe droplets relative to the time between droplet-droplet collisions. The flow profile and the natureof the emulsifier used influenced these times.

The characteristics of the different type ofhomogenizers are reported in Table 1.

High-speed mixer and colloid mills are suitablefor preparing emulsions with relatively large drop-let sizes (>1 mm), while the other types of homog-enizers can be used to prepare submicron droplets.High-speed mixers, ultrasonic homogenizers,microfluidizer, and membrane homogenizers canbe used for primary emulsification, whereas high-pressure valve homogenizers and colloid mills aremost suitable for secondary emulsification.Most ofthese homogenizers have high productivity andthey are able to work in a batch or continuousoperation mode. In particular, membrane homoge-nizers have appreciably lower productivity than theother major types of homogenizers. The use ofmembrane homogenizers may be particularly use-ful where narrow droplet size distributions areimportant such as for drug delivery.

References

Leal-Calderon F, Schmitt V, Bibette J (2007) Emulsionscience basic principles. Springer, New York

Emulsification by MembraneOperations

Emma Piacentini and Lidietta GiornoInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Emulsions can be generated using membraneoperations by: (i) direct membrane emulsification,where a microporous membrane is used to dis-perse one of two immiscible liquids as a form ofdroplets into another liquid or by (ii) premix mem-brane emulsification, where a course emulsion isextruded through the membrane pores in order togenerate fine droplet emulsion. A pressure isrequired to cause the dispersed phase to permeatethrough the membrane. If any shear is applied onthe membrane surface, droplets can be spontane-ously detached from the pore outlets on the actionof the interfacial tension when they reach a certainsize. The method is referred as static membraneemulsification (Kukizaki 2009). The shear on themembrane surface can be generated by movingthe continuous phase or using moving mem-branes. The method is referred as dynamic mem-brane emulsification. The shear stress is generatedby the continuous phase flowing tangentially tothe membrane surface (cross-flow membraneemulsification) (Williams et al. 1998) or stirred

Emulsification byMembrane Operations, Table 1 Different membrane operations used for the production emulsiondroplets

Advantages Disadvantages

Dynamic membrane emulsification

Stirred ME Suitable for low volumes of continuousphase

Emulsion production at batch scale

Useful to study the effect of differentexperimental conditions on the emulsionpreparation

The shear stress at the membrane surface isnonuniform and depends on the cell geometry

Cross-flow ME Constant shear stress at the membranesurface

Emulsion droplet breakup as a consequence ofthe recirculation

Suitable for large-scale production andcontinuous or semicontinuous operation

High dispersed phase concentration is obtainedafter a long time of operation

Cross-flow MEwith staticpromoter

Constant shear stress at the membranesurface

High dispersed phase concentration is obtainedafter a long time of operation

Suitable for large-scale production andcontinuous or semicontinuous operation

Suitable to prevent droplet breakup

Pulsed-flow ME Suitable for large-scale production andcontinuous or semicontinuous operation

–

Able to prevent droplet breakup

High dispersed phase concentrationsobtained in a single pass

Rotating ME Suitable for large-scale production andcontinuous or semicontinuous operation

Complicated and more expensive design

Higher power consumptionAble to prevent droplet breakup

High dispersed phase concentrationsobtained in a single pass

(continued)

676 Emulsification by Membrane Operations

Emulsification by Membrane Operations, Table 1 (continued)

Advantages Disadvantages

Vibrating ME Suitable for large-scale production andcontinuous or semicontinuous operation

Complicated and more expensive design

Able to prevent droplet breakup Higher power consumption

Nonuniform temporal distribution of shear stresson the membrane surface

Static membrane emulsification

Simple experimental setup Emulsion production at batch scale

Low energy input The production of uniform droplets is possibleonly at low dispersed phase flux with lowproductivity

Able to prevent droplet breakup

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(stirred membrane emulsification) (Stillwellet al. 2007). The use of static turbulence pro-moters in cross-flow membrane emulsificationallows increasing the shear stress at the membranesurface while maintaining a low value along thecircuit out of the membrane (Koris et al. 2011).Alternatively, a periodic flow pulsation is gener-ated in the continuous phase without recirculation(pulsed-flow membrane emulsification) (Holdichet al. 2013). Moving membranes can be usedinstead of the commonly used stationary mem-branes. The droplet detachment from the mem-brane surface is controlled by rotating(Vladisavljević and Williams 2006) or vibrating(Holdich et al. 2010) the membrane within anotherwise static continuous phase. Membraneoperations used for the production emulsion drop-lets are described in Table 1.

Emulsification by membrane operation is con-trolled by:

• Membrane parameters, including porosity,mean pore size, pore geometry, pore distance,and membrane surface wettability

• Phase parameters, including interfacial ten-sion, emulsifier type and concentration, viscos-ity and density of dispersed and continuous

phases, phase composition, pH, and ionicstrength

• Process parameters, including wall shearstress, transmembrane pressure, membranemodule configuration, and temperature

Comparing to the conventional emulsificationprocesses, emulsification by membrane operationpermits to obtain a better control of droplet sizeand droplet size distribution, low energy andmaterial consumption, and modular and easyscale-up. Nevertheless, productivity (m3/day) ismuch lower, and therefore the challenge in thefuture is the development of new membranesand innovative membrane operations to keep theknown advantages and maximize theproductivity.

References

Holdich RG, Dragosavac MM, Vladisavljevic GT,Kosvintsev SR (2010) Membrane emulsification withoscillating and stationary membranes. Ind Eng ChemRes 49:3810–3817

Holdich RG, Dragosavac MM, Vladisavljevic GT,Piacentini E (2013) Continuous membrane emulsifica-tion with pulsed (oscillatory) flow. Ind Eng Chem Res52:507–515

678 Emulsifier

Koris A, Piacentini E, Vatai G, Bekassy-Molnar E,Drioli E, Giorno L (2011) Investigation on the effectsof a mechanical shear-stress modification method dur-ing cross-flow membrane emulsification. J Membr Sci371:28–36

Kukizaki M (2009) Shirasu porous glass (SPG) membraneemulsification in the absence of shear flow at the mem-brane surface: influence of surfactant type and concen-tration, viscosities of dispersed and continuous phases,and transmembrane pressure. J Membr Sci 327:234–243

Stillwell MT, Holdich RG, Kosvintsev SR, Gasparini G,Cumming IW (2007) Stirred cell membrane emulsifi-cation and factors influencing dispersion drop size anduniformity. Ind Eng Chem Res 6:965–972

Vladisavljević GT, Williams RA (2006) Manufacture oflarge uniform droplets using rotating membrane emul-sification. J Colloid Interface Sci 299:396–402

Williams RA, Peng SJ, Wheeler DA, Morley NC,Taylor D, Whalley M, Houldsworth DW (1998) Con-trolled production of emulsions using a crossflowmem-brane, part II: industrial scale manufacture. Chem EngRes Des 76:902–910

Emulsifier


Emulsifiers are a class of molecules with surfaceactive properties (Dickinson 2009). This behavioris due to their amphiphilic structure which

Water-in-oil emuls

Hydrophilic Hea

Emulsifier,Fig. 1 Emulsifier structureand emulsifier distributionat water-in-oil and oil-in-water emulsion interface

contains both a polar or hydrophilic head and anonpolar or hydrophobic tail (Fig. 1). A measureof the degree to which an emulsifier is hydrophilicor lipophilic is given by the hydrophilic-lipophilicbalance (HLB) determined by calculating valuesfor the different regions of the molecule. Emulsi-fiers adsorb at interfaces between two immiscibleliquid anchoring its hydrophilic part into waterand its lipophilic part into oil decreasing the inter-facial tension between them (Fig. 1). This facili-tates droplets disruption during homogenizationand, in the case of membranes, lowering the min-imum emulsification pressure. Emulsifiers havean important role in emulsion stabilization againstdroplets coalescence and/or aggregation provid-ing electrostatic repulsion, steric repulsion, and/orstrength to the interfacial layer of the droplets.

Emulsifiers are normally classified accordingto the head group type as ionic (anionic and cat-ionic), nonionic, and amphoterics (zwitterionics).Anionic emulsifiers contain anionic functionalgroups at their head, such as sulfate (sodiumdodecyl sulfate, SDS), sulfonate, phosphate, andcarboxylates. Cationic emulsifiers contain cat-ionic functional groups at their head, such aspH-dependent primary, secondary, or tertiaryamines and permanently charged quaternaryammonium cation (cetyl trimethylammoniumbromide, CTAB). Nonionic emulsifiers havepolar headgroups and they include glycol andglycerol esters, polyoxyethylene esters,

ion Oil-in-water emulsion

EMULSIFIER

d

Hydrophobic Tail

Emulsion 679

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polyoxyethylene ethers, polyoxyethylene–po-lyoxypropylene copolymers, sorbitan derivatives,and sucrose esters. Zwitterionic (amphoteric)emulsifiers have both cationic and anionic centersattached to the same molecule. The cationic part isbased on primary, secondary, or tertiary amines orquaternary ammonium cations. The anionic partcan be more variable and include sulfonates,sultaines, betaines, and phosphates (lecithin).

A variety of emulsifiers are natural productsderived from plant or animal tissue such as hydro-colloids (high molecular weight polysaccharides)and proteins.

References

Dickinson E (2009) Hydrocolloids as emulsifiers andemulsion stabilizers. Food Hydrocolloids 23:1473–1482

Emulsifiers

▶Amphiphilic Molecules

Emulsion


An emulsion consists of two immiscible liquids(usually oil and water) with one of the liquids(dispersed phase or internal) dispersed as a formof spherical droplets in the other (continuous phaseor external) (Israelachvili 1994). Depending uponthe nature of the dispersed phase, the emulsions areclassified as (i) oil-in-water emulsions (O/W)consisting of oil droplets dispersed in an aqueousphase and (ii) water-in-oil emulsions (W/O)consisting of aqueous droplets dispersed in an oilphase. It is also possible to prepare various types of

multiple emulsions, for example, water-in-oil-in-water emulsions (W/O/W), in which water dropletsare dispersed within larger oil droplets which arethemselves dispersed in an aqueous phase andoil-in-water-in-oil emulsions (O/W/O) consistingof oil droplets dispersed in larger water dropletswhich are themselves dispersed in an oil phase.

The preparation of an emulsion is termed emul-sification and the agents used for this purpose aretermed emulsifiers. Other agents, such as emul-sion promoters or stabilizers, are often added to anemulsion to promote the emulsifying process, forexample, by increasing the viscosity or providinga protective colloid action. The preparation ofemulsions involves breaking up the internalphase by supplying mechanical or chemicalenergy. When an emulsion is formed, the interfacebetween the phases is considerably increased as aresult of the droplet formation. The liquid alwaystends to reduce its surface or interface to a mini-mum; therefore, an increase in interface is possi-ble only if energy is supplied. The work that mustbe expended on drop division is:

dA ¼ gdI

where dA is the work to be expended and dI is theincrease in interface. The proportionality factor isthe interfacial tension g between the phases to beemulsified. Thus, if the interfacial tensionbetween the two phases is high, considerablemechanical energy is required for emulsificationunless an emulsifier is added; if the interfacialtension is low, little mechanical energy isconsumed.

According to the droplet size, emulsions areclassified as follows:

• Macroemulsions: these usually have a sizerange of 0.1–5 mm.

• Nanoemulsions: these usually have a sizerange of 20–100 nm.

• Micellar emulsions or microemulsions: theseusually have a size range of 5–50 nm.

If the droplet size exhibits a wide statisticaldistribution, the emulsion is described as polydis-perse, in contrast to monodisperse systems with a

http://dx.doi.org/10.1007/978-3-662-44324-8_1789

EMULSIONCREAMING

SEDIMENTATION

FLOCCULATION

No

ch

ang

e in

dro

ple

t si

ze a

nd

siz

e d

istr

ibu

tio

n

OSTWALD RIPENING

COALESCENCE

PHASEINVERSION

Ch

ang

e in

dro

ple

t si

ze a

nd

siz

e d

istr

ibu

tio

n

Emulsion, Fig. 1 Thevarious breakdownprocesses in emulsions

680 Emulsion

uniform droplet size. The ideal particle sizedepends on the available methods of preparationand industrial application in each case. Anotherimportant emulsion property is the ratio of thevolume of the dispersed phase (Vi) to the volumeof the continuous phase or (Ve) is called the phasevolume ratio (F). IfF < 0.43 (Vi = 30 % of totalvolume), the flow properties of the emulsion aredetermined primarily by the continuous phase. IfF > 0.43, the viscosity of the emulsion generallyincreases. As F increases, either phase reversal orcream formation occurs.

Emulsion stability should match its applica-tion. Thus, for a number of applications, the emul-sion should be stable under very specificconditions, but it should break after its purposehas been achieved according to a specific condi-tion (such as temperature, pH, or salts, or the like).An emulsion is stable if fusion of the droplets isprevented by a sufficiently high energy barrier(Tadros 2013). In general, the energy barrier isbuilt up by the film of emulsifier that forms atthe surface of the droplets. Several breakdownprocesses may occur on storage depending on

particle size distribution and density differencebetween the droplets and the medium (Fig. 1).

In sedimentation, the uniform dispersion of thedroplets is disturbed by aggregation, which leadsto settling or creaming of the internal phase. Thisprocess results from external forces usually grav-itational or centrifugal. When such forces exceedthe thermal motion of the droplets (Brownianmotion), a concentration gradient builds up inthe system with the larger droplets moving fasterto the top (if their density is lower than that of themedium) or to the bottom (if their density is largerthan that of the medium) of the container. To keepan emulsion stable, such aggregation must beprevented since droplet aggregates sedimentfaster than individual small droplets. Sedimenta-tion is not always necessarily accompanied bycoalescence. Although the distribution has beenaltered, the original dispersion can be restored byshaking or stirring. Flocculation refers to aggre-gation of the droplets (without any change inprimary droplet size) into larger units. It is theresult of the van der Waals attraction that is uni-versal with all disperse systems. Flocculation

Emulsion 681

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occurs when there is no sufficient repulsion tokeep the droplets apart to distances where thevan der Waals attraction is weak. Flocculationmay be “strong” or “weak,” depending on themagnitude of the attractive energy involved. Oneway to overcome the van der Waals attraction isby electrostatic stabilization using ionic surfac-tants, which results in the formation of electricaldouble layers that introduce a repulsive energythat overcomes the attractive energy. The secondand most effective method of overcoming floccu-lation is by “steric stabilization” using nonionicsurfactants or polymers. Ostwald ripening(disproportionation) results from the finite solu-bility of the liquid phases. Liquids that are referredto as being immiscible often have mutual solubil-ities that are not negligible. With emulsions,which are usually polydisperse, the smaller drop-lets will have larger solubility when comparedwith the larger ones (due to curvature effects).With time, the smaller droplets disappear andtheir molecules diffuse to the bulk and becomedeposited on the larger droplets. With time, thedroplet size distribution shifts to larger values.Several methods may be applied to reduce Ost-wald ripening: (i) Addition of a second dispersedphase component that is insoluble in the continu-ous medium. In this case, partitioning betweendifferent droplet sizes occurs, with the componenthaving low solubility expected to be concentratedin the smaller droplets. During Ostwald ripeningin a two-component system, equilibrium isestablished when the difference in chemicalpotential between different size droplets (whichresults from curvature effects) is balanced by thedifference in chemical potential resulting frompartitioning of the two components. This effectreduces further growth of droplets.(ii) Modification of the interfacial film at emulsioninterface. By using surfactants that are stronglyadsorbed at the emulsion interface (i.e., polymericsurfactants) and that do not desorb during ripening(by choosing a molecule that is insoluble in thecontinuous phase), the rate could be significantlyreduced. In coalescence, the individual dropletsfuse together. First, the smaller droplets are

absorbed by the larger droplets, and then increas-ingly larger drops merge together until two con-tinuous phases are finally formed. The drivingforce for coalescence is the surface or film fluctu-ations which results in close approach of the drop-lets whereby the van der Waals forces is strongthus preventing their separation. Two droplets canonly coalesce if the intervening layer of liquid ispierced when they approach each other. There-fore, coalescence is opposed in two ways by theemulsifier film surrounding the droplets. First, asin the case of aggregation, the like charges of theelectrical double layer prevent them fromapproaching each other. Second, the buildup ofan elastic surface film causes the emulsion drop-lets to bounce off each other when they collide.Coalescence is always followed by acceleratedsettling or creaming, which destroys the emulsioncompletely. The emulsion is then broken and can-not be reconstituted by shaking or stirring. Thedriving force for prevention of coalescence is toproduce a stable film that can be achieved by twomechanisms and their combination: (i) increasedrepulsion both electrostatic and steric and(ii) dampening of the fluctuation. In general,smaller droplets are less susceptible to surfacefluctuations and hence coalescence is reduced.This explains the high stability of nanoemulsions.The phase inversion refers to the process wherebythere will be an exchange between the dispersephase and the medium. For example, an O/Wemulsion may with time or change of conditionsinvert to a W/O emulsion. In many cases, phaseinversion passes through a transition statewhereby multiple emulsions are produced. Phaseinversion of emulsions can be one of two types:transitional inversion induced by changing thefacers that affect the HLB of the system, forexample, temperature and/or electrolyte concen-tration, and catastrophic inversion, which isinduced by increasing the volume fraction of thedisperse phase.

Emulsions have application in several indus-trial systems such as food emulsion, for example,mayonnaise, salad creams, deserts, and bever-ages; personal care and cosmetics, for example,

682 Emulsion Characterization

hand creams, lotions, hair sprays, and sunscreens;and pharmaceuticals, paints, and bitumenemulsions.

References

Israelachvili J (1994) The science and applications ofemulsions - an overview, Colloids and Surfaces A:Physicochemical and Engineering Aspects 91: 1–8.

Tadros TF (2013) Emulsion Formation, Stability, andRheology, in Emulsion Formation and Stability(ed T. F. Tadros), Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim, Germany.

Emulsion Characterization

Volker Gaukel, Heike Schuchmann andRichard BernewitzFood Process Engineering, Karlsruhe Institute ofTechnology (KIT), Karlsruhe, Germany

The production of emulsions via membrane pro-cesses is investigated since 30 years and startedwith investigations from Nakashima et al. withporous glass membranes (Nakashimaet al. 1991). There are several principles of mem-brane emulsification as, e.g., direct emulsificationand premix emulsification. But neverthelesswhich process is used, the characterization of theemulsion is an important issue.

Besides the emulsion’s ingredients, which aredefined by the manufacturer, there are many struc-ture parameters which influence the physical sta-bility, the color, the rheological behavior, or thecontrolled release properties of the emulsion.

If the kind of emulsion is not known, the firstcharacterization method must identify if it is an oilin water (o/w) or water in oil (w/o) emulsion. Inmany cases this can be easily done by a dilutiontest. This means that an o/w emulsion can bedilutedwith a hydrophilic phase andw/o emulsionscan be diluted with a hydrophobic phase. Measure-ment of conductivity of the emulsion is anotherpossibility as o/w emulsions have in general amuch higher conductivity than w/o emulsion.

These tests cannot identify if a double emulsion(e.g., oil in water in oil phase) is present. This mustbe investigated with microscopic methods.

The two main characteristics of an emulsion,which influence the physical stability, the color,and the rheological behavior, are the drop sizedistribution (DSD) and the disperse phase content(DPC) (Schuchmann 2007).

The DPC is set by the manufacturer and isconstant when using premix membrane emulsifi-cation. Using other membrane emulsification pro-cesses like cross flow devices, the DPC ofemulsions is adjusted by the rate of flow of theoil and water phase or by recirculation time of thecontinuous phase/emulsion and changes continu-ously. Regarding multiple emulsions, additionalinstability mechanisms may lead to changes ofDPC while emulsifying and storing. The DPCcan be measured with differential scanning calo-rimetry (DSC) (Schuch et al. 2013; Dalmazzoneet al. 2009), NMR (Bernewitz et al. 2011; vanDuynhoven et al. 2007), or rheological character-istics of single and double emulsions.

The DSD depends on several factors as the kindand pore size of the membrane, the transmembraneflux, the shear forces at the membrane surface, aswell as the composition of the emulsion (kind ofemulsifier, viscosity of the phases, etc.). The DSDcan be measured with various methods which arereferred here. The DSD can also change duringstorage and distribution of an emulsion leading toa product deterioration and phase separation.A rough indication for the physical stability of anemulsion can be the zeta potential (z), whereas z>30 mV indicates a stable emulsion. Anothermethod is a Dynamic Mechanical Analysis(DMA) of the emulsion (Brummer 2006).

As the changes of DSD are often very slow andcannot be monitored over the whole shelf life of aproduct, there are several test procedures to accel-erate this process. A procedure is to rise the stor-age temperature from room temperature to40–50 �C which accelerates the deterioration pro-cesses by the factor 2. Other means are the expo-sure of the emulsion to many short temperatureabuses or to a centrifugal field which can shortenthe monitoring time by the factor 10–2,000. Itshould be kept in mind that all the acceleration

Emulsion Liquid Membrane (ELM) 683

techniques may change the structure of the emul-sion and may lead to differing results compared toreal storage conditions.

E

References

Bernewitz R, Guthausen G, Schuchmann HP (2011) NMRon emulsions: characterisation of liquid dispersed sys-tems. Magn Reson Chem 49:93–104

Brummer R (2006) Most important test methods. In: Rhe-ology essentials of cosmetic and food emulsions.Springer, Berlin, pp 75–80

Dalmazzone C, Noik C, Clausse D (2009) Application ofDSC for emulsified system characterization. Oil & gasscience and technology – rev. IFP 64(5):543–555

Nakashima T, Shimizu M, Kukizaki M (1991) Membraneemulsification by microporous glass. Eng Mater61&62:513–516

Schuch A, Köhler K, Schuchmann HP (2013) Differentialscanning calorimetry (DSC) in multiple W/O/W emul-sions: a method to characterize the stability of innerdroplets. J Therm Anal Calorim 111(3):1881–1890

Schuchmann HP (2007) In: Bröckel U, Meier W,Wagner G (eds) Product design and engineering: bestpractices, vol 1. Wiley-VCH, Weinheim, p 63

van Duynhoven PM, Maillet B, Schell J, Tronquet M,Goudappel GJW, Trezza E, Bulbarello A, vanDusschoten D (2007) A rapid benchtop NMR methodfor determination of a droplet size distributions in foodemulsions. Eur J Lipid Sci Technol 109(11):1095–1103

Further ReadingMcclements DJ (2007) Critical review of techniques and

methodologies for characterization of emulsion stabil-ity. Crit Rev Food Sci Nutr 47(7):611–649

Schramm LL (1992) Emulsions – fundamentals and appli-cations in the petroleum industry, vol 237. AmericanChemical Society, Washington, DC

Emulsion Liquid Membrane (ELM)

Vladimir S. KislikCampus Givat Ram, Casali Institute of AppliedChemistry, The Hebrew University of Jerusalem,Jerusalem, Israel

Emulsion liquid membrane, ELM, is a system inthe form of double emulsions (for details, seeChakraborty et al. 2010). It may be of two types:water-in-oil emulsion dispersed in an external

aqueous phase and oil-in-water emulsion dis-persed in an outer organic phase. The membranephase in the water-in-oil-in-water (W/O/W) typeis the immiscible oil phase separating the aqueousphases, while in the O/W/O type, the immisciblewater phase separating the two organic phasesacts as the LM. Hence, the liquid membraneserves here a dual purpose: (a) permitting selec-tive transfer of one or more components through itfrom external phase to internal droplets and viceversa a`nd (b) preventing mixing of external andinternal phases. The emulsion is dispersed in thefeed solution, andmass transfer from the feed to theinternal receiving phase takes place. ELMs werefirst used for separation of hydrocarbons fromwastewater with high separation efficiency. Com-pared to conventional processes, emulsion liquidmembrane (ELM) process has some attractive fea-tures, for example, simple operation, high effi-ciency, extraction and stripping in one stage, largerinterfacial area, and scope of continuous operation.

Since an ELM is a thin film of liquid (oil oraqueous) composed of surfactants and their sol-vents between a feed and a receiving phase, anyimmiscible liquid can serve as a membranebetween two liquid or gas phases containing asolute at different concentrations. If the solute issoluble in the membrane phase and has a reason-able diffusivity through the membrane, then itsselective transport through the membrane fromhigher to lower concentration can be achieved.This type of permeation has simple mechanismand not of much technical importance.

At facilitated transport mechanism, liquidmembrane incorporates a reactive component orcarrier, reacting reversibly and selectively withspecies of interest to carry the formed complexesacross the LM to the internal phase, and dissoci-ates, discharging the solute to the internal phase.The unchanged carrier then diffuses back to themembrane-external phase interface (see Fig. 1).A small amount of carrier is required in the mem-brane phase even for achieving a high degree ofseparation. At different proton concentrations in theaqueous phase or using another ions, ion exchangeprocesses between two LM surfaces occur. Thisphenomenon is called coupled mass transport. Ifthe transports of these two different species occur

Cm

Ci

Cmc

Feed PhaseI

Feed

RH

MR2

H+

M2+

M2+

StripH+

Organic

MembranePhase II

Strip PhaseIII

H+

H+

Ci’

C’mc

Emulsion Liquid Membrane (ELM), Fig. 1 Schematicrepresentation of the liquid membrane globule and concen-tration profile of solute through ELM

684 Emulsion Liquid Membrane for Wastewater Treatment

in the same direction, it is called cotransport, whiletransport in opposite direction is calledcountertransport. Evidently the process leads tothe transport of targeted ionic species across themembrane against their concentration gradient.This so-called “uphill” transport will continueuntil one driving factor (difference of chemicalpotentials) is balanced by the difference betweenchemical potentials of another transported ion.

The ELM technique has great potential forrecovery and removal of different metal ions.Separation of metals like copper, zinc, cadmium,cobalt, nickel, mercury, uranium, chromium, rhe-nium, and several others, including noble metalslike gold and silver, lanthanides, and rare earths,was studied. To date, there are two industrialplants installed for zinc recovery from wastewaterin Austria, having a capacity of 75 m3/h and700 m3/h, removing zinc selectively from 500 to3 ppm. Bis(2-ethylhexyl) dithiophosphoric acid

has been used as carrier. One more plant with the200 m3/h capacity in Germany and the 200 m3/hcapacity plant in the Netherlands.

Weak acids like phenol and cresol and weakbases like ammonium and amines have been suc-cessfully removed from wastewater. Among them,the separation-concentration of phenol has beenintensively investigated. Phenol removal fromwastewater was commercialized in China. Phenolis removed from about 1,000 ppm to 0.5 ppm withan extraction efficiency of greater than 99.95 %.

ELM technology has been applied to a greatextent for separation of mixtures of saturated andaromatic hydrocarbons, of amino acids, and ofstrong acids like nitric acid and thiocyanate. Cya-nide removal from wastewater in gold processingis commercialized in China. Cyanide is reducedfrom about 130 ppm to 0.5 ppmwith an extractionefficiency of 99.6 %.

ELM has promise in the fields of biotechnol-ogy and biomedicine and has found application inthe separation of organic acids, fatty acids, purifi-cation of antibiotics, enzyme-catalyzed reactions,and detoxification of blood.

References

Chakraborty M, Bhattacharya C, Datta S (2010) Emulsionhybrid liquid membranes: definitions and classification,theories, module design, applications, new directions andperspectives. In: Kislik V (ed) Liquid membranes princi-ples and applications in chemical separations & waste-water treatment, 1st edn. Elsevier, Boston, pp 141–200

Emulsion Liquid Membranefor Wastewater Treatment

N. OthmanFaculty of Chemical and Energy Engineering,Universiti Teknologi Malaysia, Skudai, Johor,Malaysia

Emulsion liquid membranes are known as doubleemulsion system. The advantage of this process isextraction and stripping process occurred

Emulsion Liquid Membrane for Wastewater Treatment 685

E

simultaneously in one single-step operation, andequilibrium limitation can be removed. It also canreduce the amount of expensive extractant; highfluxes and high selectivity are possible. It is alsopossible to treat any source of wastewatercontaining organics and metals even at low con-centration. The primary emulsions of water in oilemulsion were prepared by emulsifying twoimmiscible phases of the stripping solution andorganic liquid membrane phase with a surfactantto produce an emulsion. This primary emulsion isthen dispersed in the solution or phase to betreated. Mass transfer takes place between thefeed phase and the internal phase through theliquid membrane phase. The illustration and sche-matic diagram of the process is shown in Fig. 1.The organic liquid membrane may contain a car-rier to facilitate the extraction process. The carrierwill act as a shuttle to carry the solute from exter-nal interface to the internal interface or receiving

Phase III

A

Emulsion LiquidMembranefor WastewaterTreatment, Fig. 2 Themechanism of coupletransport in emulsion liquidmembrane

Phase IIIPhase I Phase II

Emulsion Liquid Membrane for Wastewater Treat-ment, Fig. 1 A schematic diagram of emulsion liquidmembranes

phase. At the external interface, the carrier willform carrier-solute complexes and diffuse to theinternal interface and release the solute into thereceiving phase by the reaction with the strippingagent. For example, in metal separation fromwastewater, even in very low concentration, thecarrier will selectively combine with the solutes toform a metal-carrier complex, and the complexwill permeate through the membranes from theouter to the inner interface. At the inner interface,the complex decomposes by the reversal of theequilibrium reaction, and the metal ion is liberatedinto the internal phase and the regenerated carriergoes back into the membrane phase. The mecha-nism of removal and recovery of the solute/metalassisted by the carrier is illustrated in Fig. 2.

The major problem associated with ELMs inthe wastewater treatment is emulsion stability. Ifthe emulsion globules break and the inner dropletphase spills into the continuous phase, the sepa-ration is lost. Interfacial shear between the contin-uous phase and membrane phase causes the liquidmembrane to thin and, in some cases, rupture.

Another problem is osmotic swelling althoughit is rarely mentioned in the literature. This phe-nomenon occurs when water in external phasesdiffuses through the organic membrane phase andswells the inner aqueous droplet phase. Theincreased volume of the internal phase leads toincreased breakage and dilution of the concen-trated solute in the droplet phase.

For the last three decades, this method hasattracted many studies in the area of hydrometal-lurgy such as separation of metal ions either fromwastewater or from ores (Othman et al. 2006; Reisand Carvalho 1993; Reed et al. 1987; Marr 1984;

A A

Phase II

A

Membrane phase

A soluteCarrier molecule

686 Emulsion Pertraction Technology for Zinc Recovery

Draxler and Marr 1986; Babcock et al. 1986). Ithas been reported that emulsion liquid membranesystem has been successfully used to recover cop-per selectively from waste stream of mine solu-tions (Wright et al. 1995).

From the viewpoint of practical application inwastewater treating processes, recovery processesfor copper (Volkel et al. 1980; Goto et al. 1989a),uranium (Hayworth et al. 1983), and zinc (Marr1984) were examined in test plant. The first appli-cation of emulsion liquid membrane (ELM) on anindustrial scale was the process to remove zincfrom wastewater at a textile plant in Austria(Draxler and Marr 1986).

In order to get a better understanding of emul-sion liquid membrane process and the systempotentials, the extraction performance must bestudied based on the kinetics and thermodynamicaspects. The parameters that affect the soluteextractability and selectivity should be identified.The parameters such as stripping agent types andacidity that control the mass transfer of solute,volume ratio of emulsion to external phase thataffects the mass transfer area of extraction pro-cess, and carrier concentration, type of diluents,swelling, residence time, and agitation rate thatcontrol the extraction performance and thebreakup rate of emulsion should be studied.

References

Babcock WC, Friesen DT, Lachapelle ED (1986) Liquidmembranes for separating uranium from vanadium anduranium from molybdenum. J Membr Sci26(3):303–312

Draxler J, Marr R (1986) Emulsion liquid membranes. PartI: phenomenon and industrial application. Chem EngProcess 20:319–329

Goto M, Kondo K, Nakashio F (1989) Acceleration effectof anionic surfactants on extraction rate of copper withliquid surfactant membrane containing LIX65N andnonionic surfactant. J Chem Eng Jpn 22:79–98

Hayworth HC, HoWS, BurnsWA Jr, Li NN (1983) Extrac-tion of uranium from wet process phosphoric acid byliquid membranes. Sep Sci Technol 18(6):493–521

Marr R (1984) Pilot plant studies of liquid membraneseparation. Proceeding of Eng. Found. Conf. On NewDirections in Separation technology, Davos

Othman N, Mat H, Goto M (2006) Separation of silverfrom photographic wastes by emulsion liquid mem-brane system. J Membr Sci 282(1–2):171–177

Reed DL, Bunge AL, Noble RD (1987) Influence of reac-tion reversibility on continuous-flow extraction byemulsion liquid membrane. In: Noble RD, Way JD(eds) Liquid membranes: theory and applications.American Chemical Society, Washington, DC

Reis MTA, Carvalho JMR (1993) Recovery of zinc fromindustrial effluent by emulsion liquid membranes.J Membr Sci 84:201–211

Volkel W, Halwachs W, Schugerl K (1980) Copper extrac-tion by means of a liquid surfactant membrane process.J Membr Sci 6:19–31

Wright JB, Nilsen DN, Hundley G, Galvan GJ (1995) Fieldtest of liquid emulsion membrane technique for copperrecovery from mine solutions. Miner Eng 8:549–556

Emulsion Pertraction Technologyfor Zinc Recovery

Immaculada Ortiz and Eugenio BringasDepartment of Chemical and BiomolecularEngineering, University of Cantabria, Santander,Cantabria, Spain

The emulsion pertraction technology (EPT) is aseparation process that combines the ability ofliquid membranes to promote the uphill transportof target species by the coupling between masstransfer and chemical reaction and the benefits ofusing membrane contactors, namely, large inter-facial area, nondispersive contact, and indepen-dent flow of the fluid phases. Figure 1 shows theflow diagram of the EPT process that comprisestwo essential process units: a microporous hollowfiber membrane contactor (see Fig. 2) and theemulsion vessel that contains a pseudo-emulsionconsisting of the organic phase formulated with aselective organic carrier and the dispersed strip-ping solution. The target solute is chemicallytransferred from the aqueous feed to the organicphase that is embedded in the pores of the hollowfibers due to their hydrophobic character. Next,the solute-carrier complex diffuses to the interfacebetween the organic and the droplet of strippingphase where the back-extraction reaction occurs.The solute is recovered from the internal aqueousphase after emulsion settling (San Románet al. 2010; Urtiaga et al. 2010; Carrera

AC ↔ A + C— —

AC—

AC—

C—

A + C ↔ AC— —

C—A

Back-extraction

Extraction

Concentrate

RafinateFeed SolutionHollow Fiber Contactor

Emulsion Tank

Emulsion

FeedSolution

Strippingdroplet

Emulsion

Hollowfiber

Pore

A

Masstransfer

Emulsion Pertraction Technology for Zinc Recovery, Fig. 1 Performance of the EPT process

Emulsion Pertraction Technology for Zinc Recovery 687

E

et al. 2009; Bringas et al. 2006; Urtiagaet al. 2005; Klaassen and Jansen 2001). In theforthcoming section, the potential of the EPTtechnology to develop separation processes withzinc recovery is evaluated.

Recovery of Zinc from Liquid Wastesby EPT

Zinc’s electropositive nature makes it well-suitedfor use as a coating for protecting iron and steelproducts from corrosion. For this reason, the sur-face treatment industry accounts for almost halfzinc modern-day demand. The processes involvedin the surface treatment of components are pre-dominantly water-based, and thus the generationand management of complex liquid wastes is anissue of concern (Bringas et al. 2012). EPT hasbeen proven to be an efficient technology to per-form both the selective removal and recovery ofzinc from different wastes produced in the contextof surface treatment industry: (i) spent picklingacids (SPA) generated in the hot-dip galvanizingprocess (see composition in Table 1) and (ii) spentchromium-based passivation baths (SPB)

employed in zinc electroplating operations (seecomposition in Table 1).

Figures 3 and 4 depict the separation andrecovery objectives to be achieved by the appli-cation of EPT to the treatment of SPA and SPB.

Under the usual composition of SPA, zincforms anionic chlorocomplexes (ZnCl4

2� andZnCl3

�) while iron is present in the form of neu-tral or cationic compounds (Regel et al. 2001).Tributyl phosphate (TBP) and water are reportedas the most suitable extraction and back extractionreagents enabling the selective separation andconcentration of zinc with minimum iron extrac-tion (Cierpezewski et al. 2002). On the other hand,the information provided by the equilibrium iso-therms depicted in Fig. 5 confirms the commercialselective carrier bis(2,4,4- trimethylpenthyl)phosphinic acid (Cyanex272) as a suitable reagentto formulate the liquid membrane due to its capac-ity to selectively separate Fe3+ and Zn2+ (trampions) from chromium under the typical pH condi-tions (1.8–2.5) of the passivation baths. Sulphuricacid is selected as stripping agent (Urtiagaet al. 2010).

Figure 6 shows the kinetic results obtainedwith the SPA-TBP-water and the EPT process.

Parameter/Characteristic

Fiber material

Shell material

Type of fiber

Internal diameter of thefiber

Average pore diameter

Porosity

Effective lenght

Thickness

Interfacial area

Number of fibers

Value/Description

Polypropylene

Polypropylene

240 μm

X-50

0.04 μm

40%

0.15

30 μm

1.4 m2

10200

Emulsion PertractionTechnology for ZincRecovery,Fig. 2 Characteristics ofhollow fiber contactors for abench scale EPT process

Emulsion Pertraction Technology for Zinc Recovery,Table 1 Physical-chemical properties of spent picklingacids (SPA) and spent chromium-based passivation baths(SPB)

SPA SPB

Parameter Value Parameter Value

pH �0 pH 1.8–2.5

Zn2+ (mg/L) 122,000 Zn2+

(mg/L)2500–11,780

Fe2+ (mg/L) 100,000 Fe3+

(mg/L)20–90

Cl� (mg/l) 300,000 Cr3+

(mg/L)4500–9350

Free acidity,H+ (mol/L)

1 NO3�

(mg/L)67,520


It is concluded that the extraction (EX) and back-extraction (BEX) percentages of zinc and iron,when steady state conditions (regarding zinckinetics) are reached, are respectively, 79 %(EX Zn), 98 % (BEX Zn), 23 % (EX Fe), and38 % (BEX Fe). Under these operation condi-tions, the maximum value of selectivity of zincover iron in the stripping solution is 15 kg ofZn/kg of iron (Ortiz et al. 2004; Samaniegoet al. 2006; Samaniego et al. 2007; Carreraet al. 2009; Bringas et al. 2012). On the otherhand, the efficiency of the EPT process to carryout the regeneration of real passivation baths isdemonstrated in Fig. 7 which shows reductions

FEED SOLUTION SEPARATION PROCESS SEPARATION AND RECOVERY OBJECTIVES

Spent Pickling Acids(Zn2+, HCL, Fe2+)

Emulsion PertractionTechnology

Galvanizingprocess

Electrolyticrecovery of zinc

Coagulant (FeCl3)Raffinate(FeCl2, HCl)

Stripping solution(ZnCl2, HCl)Service

water

TBP

Emulsion Pertraction Technology for Zinc Recovery, Fig. 3 Treatment of spent pickling acids byEPTwith zinc recovery

FEED SOLUTION SEPARATION PROCESS/OBJECTIVES RECOVERY OBJECTIVES

Metallic pleces from zincelectroplating: Incoming ofZn2+ and Fe3+

Spent PassivationBath

Emulsion PertractionTechnology

Regenerated passivation bath

Electroplatingprocess

Electrolyticrecovery of zinc

Stripping solution(ZnSO4)

(Cr3+)

Cyanex272H2SO4

Emulsion Pertraction Technology for Zinc Recovery, Fig. 4 Regeneration of chromium-based passivation baths byEPT with zinc recovery

0 1 2 3 4

Equilibrium pH

% E

xtra

ctio

n

chromium

zinc

iron

Selective separation

5 6 70

25

50

75

100

Selective separationEmulsion PertractionTechnology for ZincRecovery,Fig. 5 Extractionisotherms of iron, zinc, andchromium with Cyanex272

Emulsion Pertraction Technology for Zinc Recovery 689

E

00

20

40

60

80

100

120

30 60 90 120 150 180

t (min)

CZ

n -

CF

e (g

L-1

)

Fe in stripping

Fe in SPA

Zn in stripping

Zn in SPAEmulsion PertractionTechnology for ZincRecovery,Fig. 6 Evolution with timeof iron (discontinuous line)and zinc (continuous line)concentration in the SPA andstripping solution (0.5 L ofSPAwith the compositionindicated in Table 1; 1 L oforganic solution containing50 % v/v TBP in ShellsolD70; 1 L of water)

0

20

40

% E

xtra

ctio

nC

on

cen

trat

ion

(kg

m−3

)

[Zn] [Fe]

0.43 kg m−3

0.003 kg m−3

% Extraction

Stripping composition

[Cr]

60

80

100Emulsion PertractionTechnology for ZincRecovery,Fig. 7 Extractionpercentages andconcentrations in thestripping solution after 3 hof EPT regeneration of SPBusing Cyanex272 asselective extractant


in the zinc and iron contents higher than 80 %after 3 h of experimental running with analmost negligible variation of the chromiumconcentration. Furthermore, zinc is selectivelyrecovered in the stripping solution (con-centration >35 kg/m3) being the concentrationsof iron (<0.45 kg/m3) and chromium(<0.003 kg/m3) almost negligible (Urtiagaet al. 2010; Bringas et al. 2011; Dibanet al. 2011; Bringas et al. 2012).

Therefore, these results confirm the EPT pro-cess as a suitable alternative to perform the treat-ment of spent pickling acids and spent passivationbaths allowing at the same time the zinc recoveryfor further reuse as was illustrated by Figs. 3and 4.

Future Directions

Future development will require research anddevelopment activities in the following areas:

1. Determination of the optimal operational con-ditions to: (i) maximize the extraction andback-extraction kinetics and (ii) achieve max-imum values of selectivity which permit theexploitation of the different process streamsgenerated after the application of the EPTprocess.

2. Analysis of the long-term performance of theseparation process to evaluate the stability ofthe selective carrier which guarantees the pro-cess’ economic viability.

Emulsion Rupture by Membranes 691

E

References

Bringas E, San Román MF, Ortiz I (2006) Separation andrecovery of anionic pollutants by the emulsionpertraction technology. Remediation of pollutedgroundwaters with Cr(VI). Ind Eng Chem Res45:4295–4303

Bringas E, Mediavilla R, Urtiaga A, Ortiz I (2011) Devel-opment and validation of a dynamic model for regen-eration of passivating baths using membranecontactors. Comp Chem Eng 35:918–927

Bringas E, San Román MF, Urtiaga AM, Ortiz I (2012)Integrated use of liquid membranes and membranecontactors: enhancing the efficiency of L-L reactiveseparations. Chem Eng Process. doi:10.1016/j.cep.2012.11.005

Carrera JA, Bringas E, San Román MF, Ortiz I (2009)Selective membrane alternative to the recovery of zincfrom hot-dip galvanizing effluents. J Membr Sci326:672–680

Cierpezewski R, Miesiac I, Regel-Rosocka M,Sastre AM, Szymanowski J (2002) Removal of zinc(II) from spent hydrochloric acid solutions fromzinc hot galvanizing plants. Ind Eng Chem Res41:598–603

Diban N, Mediavilla R, Urtiaga A, Ortiz I (2011)Zinc recovery and waste sludge minimization fromchromium passivation baths. J Hazard Mater192:801–807

Klaassen R, Jansen AE (2001) The membrane contactor:environmental applications and possibilities. EnvironProg 20:37–43

Ortiz I, Bringas E, San Román MF, Urtiaga AM(2004) Selective separation of zinc and iron fromspent pickling solutions by membrane-based solventextraction. Sep Sci Technol 39:2441–2455

Regel M, Sastre AM, Szymanowski J (2001) Recoveryof zinc(II) from HCl spent pickling solutionsby solvent extraction. Environ Sci Technol 35:630–635

Samaniego H, San RománMF, Ortiz I (2006) Modelling ofthe extraction and back-extraction equilibria of zincfrom spent pickling solutions. Sep Sci Technol41:757–769

Samaniego H, San Román MF, Ortiz I (2007) Kinetics ofzinc recovery from spent pickling effluents. Ind EngChem Res 46:907–912

San Román MF, Bringas E, Ibáñez R, Ortiz I (2010)Liquid membrane technology: fundamentals andreview of its applications. J Chem Technol Biotechnol85:2–10

Urtiaga A, Abellán MJ, Irabien JA, Ortiz I (2005) Mem-brane contactors for the recovery of metallic com-pounds. Modelling of copper recovery from WPOprocesses. J Membr Sci 257:161–170

Urtiaga A, Bringas E, Mediavilla R, Ortiz I (2010)The role of liquid membranes in the selectiveseparation and recovery of zinc for the regenerationof Cr(III) passivation baths. J Membr Sci 356:88–95

Emulsion Rupture by Membranes

Jose CocaDepartment of Chemical and EnvironmentalEngineering, University of Oviedo, Oviedo,Spain

Emulsions are homogeneous mixtures that consistof a dispersed phase (oil droplets in O/W emul-sions) distributed uniformly in a continuous phase(water). Ultrafiltration (UF) and microfiltration(MF) membranes have been used for the treatmentof O/W emulsions produced in industries such assteel works, metal finishing, pharmaceuticals,cosmetics, food, etc. Oil droplets are recoveredas retentate and the aqueous phase permeates themembrane. The production of 1 t of steel maydemand up to 200 t of water, the largest waterusage corresponding to the rolling mills. Mem-brane performance diminishes when oil dropletsfall below the micron range and problems becomemore pronounced when ionic surfactants are pre-sent, as they increase repulsive forces betweendroplets.

Demulsification is commonly achieved bychemical or electrostatic methods. Chemicaldemulsification destabilizes the disperse phaseby two mechanisms: coagulation (inorganicsalts) and flocculation (organic polymers).The disadvantage of chemical demulsificationis that the bulk phase has to be further treatedbefore discharge. Electrostatic demulsificationis ineffective for emulsions with high watercontent and sparking during treatment maygenerate new compounds from the surfactantand the oil. However, membranes can be usedas coalescers by forcing the emulsion throughthe pores of the membrane (Kajitvichyanukulet al. 2011).

Hydrophilic membranes induce coalescenceof O/Wemulsions while hydrophobic membranescan be used for the demulsification of W/O emul-sions (Fig. 1). The main factors affecting mem-brane demulsification are (Daiminger et al. 1995;Hong et al. 2003; Kocherginsky et al. 2003):

qO/W < 90∞

qO/W > 90∞

Hydrophobic membrane

Hydrophilic membrane

Oil

Oil

Emulsion Rupture by Membranes, Fig. 1 Influence ofmembrane characteristics in oil droplet rejection

692 Emulsion Separation by Membranes

• Membrane pore size, transmembrane pressure(DP), and imposed in-pore shear rates affectcoalescence. The smaller the membrane poresize and the lower DP, the betterdemulsification efficiency. However, smallpores coupled with a low DP would lead tolow permeation flux.

• If the DP is below a critical value (DPc), theemulsion rejection can be maximized. Above aDPc the membrane acts as a coalescer and thedroplets wet the membrane enabling the oildroplets to coalesce.

• Membrane demulsification seems to be inde-pendent of the initial disperse phase concentra-tion and membrane thickness.

• The demulsification process is determined byinteractions between droplets and membranesurface.

So far, membranes as coalescers do not showgreat advantages with respect to conventionalcoalescing techniques, e.g., fiber-bed coalescenceand electrostatic coalescers. New chemicallytreated ceramic membranes and silicon carbidesupports have a great potential for oil-water appli-cations since they are abrasion and solvent

resistant, easy to clean by backflushing, andhave a low cost and longer life than the previousgeneration membranes.

References

Daiminger U, Nitsch W, Plucinski P, Hoffmann S (1995)Novel techniques for oil/water separation. J Membr Sci99:197–203

Hong A, Fane AG, Burford R (2003) Factors affectingmembrane coalescence of stable oil-in-water emul-sions. J Membr Sci 222:19–39

Kajitvichyanukul P, Hung Y-T, Wang LK (2011) Mem-brane technologies for oil–water separation. In: WangLK, Chen JP, Hung Y-T, Shammas NK (eds) Handbookof environmental engineering, vol 13, Membraneand desalination technologies. Springer, Dordrecht,pp 639–668

KocherginskyNM,TanCL, LuWF (2003)Demulsificationof water-in-oil emulsions via filtration through ahydrophilic polymer membrane. J Membr Sci 220:117–128

Emulsion Separation by Membranes


Separation of oily emulsions from wastewaters isgenerally carried out by a combination of chemi-cal and mechanical methods. Chemicals(coagulants and flocculants) are used to destabi-lize the oil-water interface allowing the oil drop-lets to coalesce. Mechanical methods includegravity-based settlers, skimmers, dissolved air flo-tation (DAF), centrifuges, electro-coalescers, etc.

Pressure-driven membrane processes(microfiltration MF, ultrafiltration UF, and reverseosmosis RO) may be used to separate oil andwater phases. The water molecules move throughthe membrane (permeate) and a concentratedemulsion (retentate or concentrate) is obtained.Some components of the emulsion may alsomove through the membrane, depending on theircharacteristics and size and the nature of the

Oily wastewater Free oil removal

Equalizationtank

Settleable solids

Processtank

Final concentratedisposal

Filter

Oil-free permeatedischarge

UF

Concentrate

Centrifugalseparator

Emulsion Separation by Membranes, Fig. 1 Hybrid UF system for oily wastewater treatment

Emulsion Treatment with Membranes 693

E

membrane, leading to contamination of the per-meate. Fouling and scaling of the membrane leadto a poor performance and membrane cleaningprocedures must be taken into account.

When MF is used for oil-water separation,particulates, emulsified oil, and microbial contam-inants can be removed. Cross-flow velocities mayrange between 3 and 6 m/s and transmembranepressures (TMPs) between 100 and 770 kPa. Themain limitation of membrane processes is thedecline of the permeate flux over time becauseof concentration polarization, membrane fouling(due to surfactant or oil adsorption on the porewalls), gel layer formation, pore blocking by oildroplets, pH, and temperature.

Often it is not possible to use a simple mem-brane system to perform an oil-water separation:some effluents may cause severe membrane foul-ing and pretreatment is necessary to maintain ahigh and steady flux. In these situations, inte-grated-membrane or membrane-based hybridprocesses may be suitable alternatives to obtaingood process performance and to extend mem-brane life (Coca et al. 2013). A typical membranehybrid process for oily wastewaters is shown inFig. 1.

Usually the process starts with the removal ofsettleable solids and free-floating oil prior tomembrane treatment, mainly UF. This can beaccomplished in a tank with free-oil removalequipment, such as a skimmer, or by a rotatingbrush strainer, a pressure or vacuum filter toremove solids, and a centrifugal separator or ahydrocyclone to remove oil and solids. Theremaining oily wastewater (mainly stable O/W

emulsion) is then transferred to a process tankand pumped through the UF unit to remove theemulsified oil. The retentate containing the oil isrecycled to the process tank, and the permeate iscontinuously withdrawn. This process is com-monly used in the automobile industry (Cheryanand Rajagopalan 1998).

References

Cheryan M, Rajagopalan N (1998) Membrane processingof oily streams. Wastewater treatment and waste reduc-tion. J Membr Sci 151:13–28

Coca J, Gutiérrez G, Benito JM (2013) Treatment of oilywastewater by membrane hybrid processes. In: Coca-Prados J, Gutiérrez-Cervelló G (eds) Economic sustain-ability and environmental protection in mediterraneancountries through clean manufacturing methods.Springer, Dordrecht, pp 35–61

Emulsion Treatmentwith Membranes


An emulsion is a homogeneous mixture of twoimmiscible liquids, with one of them (typically oil)dispersed as droplets into the other. Emulsion drop-lets usually range between 0.1 and 20 mm in diam-eter. The two main categories of emulsions are

Oily

wastewaterFree oil

Feedtank

Settleable solids

Processtank

Concentratedisposal

Centrifugation

Oil-free permeate(to sewer)

UF

Chemical addition(coagulants/flocculants)

Emulsion Treatment with Membranes, Fig. 1 UF process for oily wastewater treatment

694 Emulsion Treatment with Membranes

oil-in-water (O/W, >30 % water) and water-in-oil(W/O,<25%water): water and highly polar liquidsare hydrophilic, while nonpolar liquids are consid-ered “oils”. Emulsions consist of two phases: aninternal or discontinuous phase (finally divideddroplets) and an external or continuous phase(which keeps the droplets in suspension), whichare bound together at the interphase. A surfactantreduces the interfacial tension between the twophases binding them together. Industrial oily waste-waters can be classified as: free oil (Dp �150 mm),dispersed oil (Dp = 20–150 mm), stable emulsifiedoil (Dp � 20 mm), and dissolved oil (Dp �5 mm).Free oils and dispersed O/W emulsions can beremoved by mechanical methods such as gravitysettling, skimming, coalescence, centrifugation,etc. (Alther 1998; Stewart and Arnold 2008). Stableemulsified oils and dissolved oils cannot beremoved efficiently by conventional methodsbecause of the small droplet size and low oilconcentration.

Membrane processes are increasingly beingapplied for treating O/W emulsions due to theiradvantages: high-quality permeate and removalefficiency, lower capital costs than with thermalprocesses, and compact design. Some of the mostpromising membrane O/W treatments are dehy-dration of emulsions by pervaporation, reverse

osmosis (RO), flocculation followed bymicrofiltration (MF), MF, membrane distillation,nanofiltration (NF), and ultrafiltration(UF) (Cheryan 1998; Chakrabarty et al. 2010).

A typical UF-based system for oily wastewa-ters, operated in a semi-batch recycle mode, isshown in Fig. 1. The final concentrate volumemay be only 3–5 % of the initial oily wastewatervolume. The system must be cleaned after a cer-tain time to restore the permeate flux. O/W emul-sions may be reduced by 85–90 % by volume andwith an oil concentration in the retentate of70–75 %.

MF or UF processes cannot remove dissolvedoil components in water. For that purpose, othermethods, such as RO or NF, are required. Thechoice of membrane material is important: inor-ganic membranes are chemically robust andexpensive; polymeric membranes have lowerresistance to aggressive feeds and are more sus-ceptible to fouling, but are considerably cheaper.In spite of the fact that effluent oil concentrationsof 5 ppm or less can be achieved with membranes,they have not found wide practical applications sofar. Membrane systems suffer from concentrationpolarization and fouling problems that lead to asubstantial flux decline with time. Membranesmust be replaced every 3–5 years.

Emulsions’ Drop Size Distribution, Measurement of 695

O/W emulsions can sometimes be treated by acombination of two membrane processes (e.g.,UF/NF, UF/RO or NF/RO) to obtain a high-quality water effluent.

E

References

Alther G (1998) Put the breaks on wastewater emulsions.Chem Eng 105:82–88

Chakrabarty B, Ghoshal AK, Purkait MK (2010) Cross-flow ultrafiltration of stable oil-in-water emulsion usingpolysulfone membranes. Chem Eng J 185:447–456

Cheryan M (1998) Ultrafiltration and microfiltration hand-book, 2nd edn. Technomic, Lancaster

Stewart M, Arnold A (2008) Emulsions and oil treatingequipment: selection, sizing and troubleshooting. GulfProfessional Publishing, Burlington

Emulsions’ Drop Size Distribution,Measurement of

Volker Gaukel, Richard Bernewitz andHeike SchuchmannFood Process Engineering, Karlsruhe Institute ofTechnology (KIT), Karlsruhe, Germany

The drop size distribution (DSD) of an emulsionhas influence on the physical stability, the color,and the rheological behavior of the emulsion and istherefore an important means of characterization.

The DSD can be measured with variousmethods. In principle one can distinguish betweenmethods which measure physical characteristicsof single drops or physical characteristics of thebulk emulsion. The latter are often used for thecharacterization of an emulsion in terms of qualitycontrol where it is only necessary to notice differ-ences between a reference and a product or whereit is sufficient to attain the DSD results only after acalibration against a system with known DSD.Examples are rheological behavior, dielectricspectrometry, dynamic scanning calorimetry,focused beam reflectance, or dynamic reflectionmeasurements. An advantage of these methods is

often that they are fast and may be used online orat least without diluting the sample.

For the measurement of DSD, a physical char-acteristic which is connected to the drop size of asingle drop-like sedimentation velocity, diffrac-tion of light at the drop surface, projected area ina microscopic image, etc., is necessary. Thismakes clear that the basis of the calculation ofDSD varies between different methods, and oneshould not expect the same DSD results fromdifferent measuring methods. In addition duringthe calculation procedure, it is partly necessary tomake assumptions and simplifications, and there-fore DSD results are very sensitive to the calcula-tion parameters which are set by the manufactureror which can be set by the user of the equipment.

The most common techniques for DSD mea-surement are presented in Table 1 which shows themeasuring principle as well as the analyzable dropsize range and some restrictions of the method.

Progress in the field of emulsions has evolvedcomplex structures, like multiple emulsions(Muschiolik 2007; Jiménez-Colmenero 2013).As in a multiple emulsion, there are more thanone DSD to determine the complex structure thatchallenges the common measuring techniques.However, progress in the field of DSD determina-tion of double emulsions has been made. Espe-cially PFG-NMR and IA of confocal laserscanning microscopy images offer possibilitiesfor the characterization of double emulsions(Schuster et al. 2012).

Another important issue in the context of DSDmeasurements is the illustration and interpreta-tion. DSDs are statistical distributions and can beshown as cumulative or density distributions.Especially showing the latter, it is very importantto consider all the rules of their calculation. To theauthors’ knowledge, there are many measurementdevices with very weak software concerning thispoint. For simplification and interpretation ofresults, it is very common to show only meanvalues of the DSD as, e.g., the Sauter Mean Diam-eter or statistical values like the median or modalvalue. Some insights on this topic are given in(Hess 2004; Sommer 2001).

Emulsions’ Drop Size Distribution, Measurement of, Table 1 Overview on the most important measurementtechniques for drop size analytics in emulsions

Method

Physicalcharacteristic forsize measurement Size range Additional information

Sedimentation Sedimentationvelocity of asingle drop

(50 nm)1 mm–1 mm

Range for the use of centrifuges in brackets. Forsmall drops, very high dilution necessary

(Statistical) imageanalytics (IA)

Projected area (0,1 nm)1 mm–20 mm

Size range depending on the image source.Electron microscopy in brackets. Information onstructure and double emulsion detection possible.High number of drops necessary for reliablestatistical analysis

Laser diffraction (LD) Diffraction oflight at the dropsurface

(50 nm)1 mm–2 mm

In brackets: with additional light sources andscattering information, necessity of complexrefraction index. High dilution necessary

Dynamic laser lightscattering (DLS)

Diffusion rate 1 nm–1 mm Dispersed phase content up to 10 % possible.Drop sedimentation leads to measurement error

Pulsed field gradientnuclear magneticresonance (PFG-NMR)

Coefficient ofdiffusion

0,2–100 mm Measurement without dilution possible.Characterization of some parameters of doubleemulsions

696 Enantiocatalytic Membrane

References

Hess WF (2004) Representation of particle size distribu-tions in practice. Chem Eng Technol 27(6):624–629

Jiménez-Colmenero F (2013) Potential applications ofmultiple emulsions in the development of healthy andfunctional foods. Food Res Int 52(1):64–74

Muschiolik G (2007) Multiple emulsions for food use.Curr Opin Colloid Interface Sci 12(4–5):213–220

Schuster S, Bernewitz R, Guthausen G, Zapp J, GreinerAM, Köhler K, Schuchmann HP (2012) Analysis ofW1/O/W2 double emulsions with CLSM: statisticalimage processing for droplet size distribution. ChemEng Sci 81:84–90

Sommer K (2001) 40 years of presentation particle sizedistributions – yet still incorrect? Part Part Syst Charact18(1):22–25

Enantiocatalytic Membrane

Mohamad Hekarl UzirSchool of Chemical Engineering, Universiti SainsMalaysia, Penang, Malaysia

Enantiocatalytic membrane is a type of mem-brane immobilized with a particular catalyst(mainly enzyme) onto the shell side for the pur-pose of carrying out an in situ reaction-separation

process of a particular enantiomer from a racemicmixture. The most popular reaction using thetechnique is esterification, where Candida rugosalipase or Candida antarctica lipase is the biocat-alyst for the reaction (Giorno et al. 2007; Lauet al. 2010). The application of membranes, espe-cially in the production of drugs and other finechemicals, has become a trend in recent years andit will be one of the best techniques for the bulksynthesis of such compounds (Lau et al. 2010; Liet al. 2003). Hollow fiber membrane with a par-ticular molecular weight cutoff is the best choiceto use as the immobilization matrix.

References

Giorno L, D’Amore E, Drioli E, Cassano R, Picci N (2007)Influence of -OR ester group length on the catalyticactivity and enantioselectivity of the free lipase andimmobilized in membrane used for the kinetic resolu-tion of naproxen esters. J Catal 247:194–200

Lau SY, Uzir MH, Kamaruddin AH, Bhatia S (2010)Lipase-catalysed dynamic resolution of racemic ibu-profen ester via hollow fibre membrane reactor: model-ling and simulation. J Membr Sci 357(1-2):109–121

Li N, Giorno L, Drioli E (2003) Effect of immobilizationsite and membrane materials on multiphasicenantiocatalytic enzyme membrane reactors. In: Li N,Drioli E, Ho W, Lipscomb G (eds) Advanced mem-brane technology. New York Academy of Sciences,New York, pp 436–452

Enantiomers 697

Enantiomer Discrimination(Enantioselectivity)


E
Enantiomer discrimination (or chiral discrimina-tion) is a method used to distinguish between twoenantiomers for the purpose of separating thecompounds. The terms is also related to enantios-electivity where it refers to the discrimination of agiven reactant A when it reacts with two alterna-tive reactants B and C or in two different ways(probably at two different sites) with a reactantB (McNaught and Wilkinson 1997). A number ofmethods have been established to undergo enan-tiomer discrimination; these include mass spec-troscopy (Czerwenka and Lindner 2004;Czerwenka et al. 2004), nuclear magnetic reso-nance (NMR) (Gafni et al. 1998; Molabaasi andTalebpour 2011), and crystal recognition(Ballesteros et al. 1995; Huai et al. 2006; Klaholzet al. 2000). These methods have been specificallydeveloped based on a particular compound withits own characteristics of crystal structure.
C

CH3

HO H

C2H5

C

CH3

H OH

C2H5

Enantiomers, Fig. 1 Structural formula of 2-butanolshowing two different enantiomers

References

Ballesteros E, Gallego M, Valcarcel M, Grases F (1995)Enantiomer discrimination by continuous precipitation.Anal Chem 67(18):3319–3323

Czerwenka C, Lindner W (2004) Enantiomer discrimina-tion of peptides by tandem mass spectrometry: influ-ence of the peptide sequence on chiral recognition.Rapid Commun Mass Spectrom 18:2713–2718

Czerwenka C, Maier NM, Lindner W (2004) Enantiomerdiscrimination by mass spectrometry: noncovalentinteraction of an N-derivatized dipeptide with variouscinchona alkoloid derivatives and comparison withenantioselective liquid-phase separations. AnalBioanal Chem 379:1039–1044

Gafni A, Cohen Y, Kataky R, Palmer S, Parker D (1998)Enantiomer discrimination using lipophilic cyclodex-trins studied by electrode response, Pulsed-GradientSpin-Echo, (PDSE) NMR and relaxation ratemeasurements. J Chem Soc Perkin Trans 2(1):19–24

Huai Q, Sun Y, Wang H, MacDonald D, Aspiotis R,Robinson H, Huang Z, Ke H (2006) Enantiomer

discrimination illustrated by high resolution crystalstructure of type 4 phosphodiesterase. J Med Chem49(6):1867–1873

Klaholz B,Mitschler A, BelemaM, Zusi C,Moras D (2000)Enantiomer discrimination illustrated by high-resolutioncrystal structures of the human nuclear receptor hRARg.Proc Natl Acad Sci USA 97(12):6322–6327

McNaught AD,Wilkinson A (Eds.), (1997) IUPAC - Com-pendium of Chemical Terminology, Blackwell Science,Oxford, England, UK

Molabaasi F, Talebpour Z (2011) Enantiomeric discrimi-nation and quantification of the chiral organophospho-rus pesticide fenamiphos in aqueous samples by a noveland selective 13P nuclear magnetic resonance spectro-scopic method using cyclodextrin as chiral selector.J Agric Food Chem 59(3):803–808

Enantiomers


Enantiomers are types of stereoisomer whosestructures are mirror image of each other, andthey are not superimposable. The property is spe-cial to most chiral compounds and exists bothnaturally or from a particular chemical synthesis(Solomons and Fryhle 2004). It represents theintrinsic property of the building block of life,which consists of amino acids, sugars, peptides,proteins, and polysaccharides (Maier et al. 2001).The simplest compound that exhibits enantio-meric property is 2-butanol with the structuralformulae given in Fig. 1.

The existence of enantiomers of a particularcompound can be determined when there is amolecule containing one tetrahedral atom withfour different groups attached to it, as shown inthe two structures of 2-butanol above (McMurry

698 Enantiomers Production by Membrane Operations

2004). Specialized drugs are mainly made of com-pounds that exhibit enantiomeric properties; theseinclude (S)-ibuprofen, (S)-naproxen, (S)-citalopram, and (+)-norcisapride. As can be appar-ently seen, only one type of enantiomer acts as anactive compound compared to theircorresponding structures. This shows that the syn-theses of such drugs are somewhat important, inparticular, for use in pharmaceutical and medicalsectors. Some of the processes carried out toobtain the compounds are tedious and require anumber of reagents as well as different catalysts.This will then lead to high cost of production(Maier et al. 2001; Tramper 1996).

References

Maier NM, Franco P, Lindner W (2001) Separation ofenantiomers: needs, challenges, perspectives.J Chromatogr A 906(1):3–33

McMurry J (2004) Organic chemistry. Thomson Books/Cole, Belmont

Solomons TWG, Fryhle CB (2004) Organic chemistry.Wiley, New Jersey

Tramper J (1996) Chemical versus biochemical conver-sion: when and how to use biocatalysts. BiotechnolBioeng 52(1):290–295

Enantiomers Production byMembrane Operations


Enantiomer production is a process of formingenantiomer of interest through membrane separa-tion. The current method of kinetic resolution andin situ membrane separation is a highly effectiveprocess which requires less energy and short reac-tion time (Lau et al. 2011). Most of the worksrelated to product separations involved lipaseenzyme immobilized onto the membrane matri-ces. These include the production of naproxenester (Giorno et al. 2003, 2007) and ibuprofenester (Lau et al. 2010).

References

Giorno L, Li N, Drioli E (2003) Use of stable emulsion toimprove stability, activity and enantioselectivity oflipase immobilized in a membrane reactor. BiotechBioeng 84(6):677–685

Giorno L, D’Amore E, Drioli E, Cassano R, Picci N (2007)Influence of -OR ester group length on the catalyticactivity and enantioselectivity of the free lipase andimmobilized in membrane used for the kinetic resolu-tion of naproxen esters. J Catal 247:194–200

Lau SY, Uzir MH, Kamaruddin AH, Bhatia S (2010)Lipase-catalysed dynamic resolution of racemic ibu-profen ester via hollow fibre membrane reactor: model-ling and simulation. J Membr Sci 357(1–2):109–121

Lau SY, Fadzil NG, Kamaruddin AH, Bhatia S, Uzir MH(2011) Conceptual design and simulation of a plant forthe production of high purity (S)-ibuprofen acid usinginnovative enzymatic membrane technology. ChemEng J 166(2):726–737

Enantiomers Separation byMembrane Operations


Enantiomers separation is a process of separat-ing isomers in a racemic mixture into theirindividual enantiomer. The use of membranes inchiral separation can be divided into twomain categories, namely, adsorption-typeenantioselective membranes and membrane-assisted resolution with non-enantioselectivesolid membranes (Pirkle and Bowen 1994; Xieet al. 2008). The former is entirely based on thetransport mechanism with an aid of chiral carriers.The work of Gumi and co-workers reported adetail account of the transfer of the (S)-propranolol attached to the N-hexadecyl-L-hydroxyproline through the formation of ionicpair, which is then being transported through thepolysulfone-based membrane (Gumi et al. 2005).The latter is a technique mostly coupled withenzymes as biocatalysts. The enzyme is initiallyimmobilized onto the shell side of the membranematrix before carrying out the required resolution,

Enantioselective Separations, Membrane Operations 699

either kinetic resolution or dynamic kinetic resolu-tion. For example, the work of Long andco-workers successfully utilized Candida rugosalipase as the source of biocatalyst for the kineticresolution of (S)-ibuprofen ester (Long et al. 2005).

E

References

Gumi T, Ferreira Q, Viegas R, Crespo J, Coelhoso I,Palet C (2005) Enantioselective separation of propran-olol by chiral activated membranes. Sep Sci Technol40(4):773–789

Long W, Kow P, Kamaruddin A, Bhatia S (2005) Compar-ison of kinetic resolution between two racemic ibupro-fen enters in an enzymatic membrane reactor. ProcessBiochem 40(7):2417–2425

Pirkle W, BowenW (1994) Preparative separation of enan-tiomers using hollow-fibre membrane technology. Tet-rahedron Asymmetry 5(5):773–776

Xie R, Chu L-Y, Deng J-G (2008) Membranes and mem-brane processes for chiral resolution. Chem Soc Rev37:1243–1263

Enantioselective Membrane


Enantioselective membrane is a type of membraneused to separate chiral compounds into their indi-vidual enantiomer. The usual type of membraneused for separation purposes is of polymeric type(Ceynowa 1998; Higuchi et al. 2010; Xieet al. 2008). The mechanism of chiral separationcan be categorized into two different types ofmembranes, namely, diffusion-selective mem-branes and sorption-selective membranes.Diffusion-selective type of membranes are basi-cally made of polymer without any specific chiralselectors (i.e., the polymer itself has chiral prop-erties); however, sorption-selective membranescan be designed in such a way that a particularchiral selector can be immobilized can beimmobilized onto the surfaces of the polymermembranes, which later gives less selective

diffusion, but provides highly selective sorption(Hadik et al. 2002; Higuchi et al. 2010).

References

Ceynowa J (1998) Separation of racemic mixtures bymembrane methods. Chem Anal 43(6):917–933

Hadik P, Szabo L, Nagy E (2002) D, L-lactic acid and D,L-alanine enantio separation by membrane process.Desalination 148(1–3):193–198

Higuchi A, Tamai M, Ko Y-A, Tagawa Y-I, Wu Y-H,Freeman B, Bing J-T, Chang Y, Ling Q-D (2010) Poly-meric membranes for chiral separation of pharmaceuti-cals and chemicals. Polym Rev 50(2):113–143

Xie R, Chu L-Y, Deng J-G (2008) Membranes and mem-brane processes for chiral resolution. Chem Soc Rev37:1243–1263

Enantioselective Separations,Membrane Operations


Enantioselective separations refer to separationmethods used to separate racemic mixture intoindividual enantiomer with the aid of membranematrix/module. In order to measure the level ofselectivity, the percent enantiomeric excess (% ee)of a mixture needs to be determined (see enantios-electivity). Membranes have been successfullyused in chiral separation with various types aswell as with different abilities: direct separationtype of membrane made from polymer/liquid andseparation with nonselective membrane assistedby chiral carriers/support (Maier et al. 2001;Pirkle and Bowen 1994).

References

Maier NM, Franco P, Lindner W (2001) Separation ofenantiomers: needs, challenges, perspectives.J Chromatogr A 906(1):3–33


700 Enantioselective Synthesis

Enantioselective Synthesis

Mohamad Hekarl UzirSchool of Chemical Engineering, Universiti SainsMalaysia, Nibong Tebal, Penang, Malaysia

Enantioselective synthesis is a chemical/bio-chemical reaction where only one enantiomer ofa chiral product is preferentially formed. The syn-thesis can be divided into two different types,namely, catalytic synthesis and biocatalytic trans-formation. Nowadays, chemical catalysis hasbeen continuously improved, especially in termsof yields and percentage enantiomeric excesses(%e.e), for instance, the synthesis of spirocyclicoxindolo-b-lactam, a combination of indole andb-lactam, which acts as antibacterial, antiviral,and antifungal agents (Zhang et al. 2014). Thestructure of spirocyclic oxindolo-b-lactam orcommercially known as chartelines is given inFig. 1.

Zhang and coworkers reported that the con-struction of chartelines started from the bifunc-tional N-heterocyclic carbine undergoingStaudinger reaction of ketenes with isatin-derivedketimines. For such a highly complex compound,the %e.e reached up to 95 % with the yield of89 %. Similar goes to the synthesis of g-, d-, ande-chiral-1-alcoloids, reported by Xu andcoworkers (2014).The reaction was based on

N

O

N

NH

N

Br

ClR1

R2

Br

Compound A: R1=R2=BrCompound B: R1=Br, R2=HCompound C: R1=R2=H

Enantioselective Synthesis, Fig. 1 Chemical structureof spirocyclic oxindolo-b-lactam

both chemical and biological catalyses utilizingZr for the carboalumination reaction and laterapplying lipase originated from Pseudomonascepacia for the acetylation reaction. However,by comparing both types of reactions, biocatalysishas been known to provide better yield as wellas %e.e (Tsai and Dordick 1996). One of therecent works in the development of pharmaceuti-cally active compounds has been reported byWenand colleagues where the group managed to syn-thesize (S)-propranolol through a coupled reac-tion with kinetic resolution as a recycling routeutilizing Candida antarctica lipase B. The resultshowed an improve %e.e up to 100 % (Wenet al. 2014).

References

Tsai S-W, Dordick JS (1996) Extraordinary enantiospe-cificity of lipase catalysis in organic media induced bypurification and catalyst engineering. BiotechnolBioeng 52(2):296–300

Wen Y, Hertzberg R, Gonzalez I, Moberg C (2014) Minorenantiomer recycling: application to enantioselectivesynthesis of beta-blockers. Chem-A Eur J 20(13):3806–3812

Xu S, Oda A, Kamada H, Negishi E (2014) Highlyenantioselective synthesis of g-, d-, and e-chiral 1-alkanols via Zr-catalyzed asymmetric carboalu-mination of alkenes (ZACA)-Cu- or Pd-catalyzedcross-coupling. Proc Natl Acad Sci U S A 111(23):8368–8373

Zhang H, Gao Z, Ye S (2014) Bifunctional N-heterocycliccarbene-catalyzed highly enantioselective synthesis ofspirocyclic oxindolo-beta-lactam. Org Lett 16(11):3079–3081

Enantioselective Synthesis byMembrane Operations


Enantioselective synthesis by membrane opera-tions refers to chemical reactions to produce oneenantiomer from a racemic mixture utilizing

Enantioselective Transport of Amino Acids by Membrane Operations 701

E

membrane as a medium of separation. The processoccurs in situ, where the product formed from thereaction will be directly separated into therequired enantiomeric compound (Giornoet al. 1995; Long et al. 2005). Another enantio-meric synthesis, which applied the hollow-fibermembrane module, is from the work of Hadik andco-workers. The polymeric membrane madefrom polypropylene was used together with achiral selector, N-3,5-dinitrobenzoyl-L-alanineoctylester, initially dissolved in toluene (Hadiket al. 2002). Hollow-fiber membrane has alsobeen applied with different types of chiral selec-tors for large-scale separation processes. Thereported results suggested that N(1-naphthyl)-leucine provided a wide range of enantiomericseparation of amino acid derivatives with %ee ofup to 95 % (Pirkle and Bowen 1994).

References

Giorno L, Molinari R, Drioli E, Bianchi D, Cestic P (1995)Performance of a biphasic organic/aqueous hollowfibre reactor using immobilized lipase. J Chem TechnolBiotechnol 64(4):345–352

Hadik P, Szabo L, Nagy E (2002) D, L-lactic acid and D,L-alanine enantio separation by membrane process.Desalination 148(1–3):193–198

Long W, Kow P, Kamaruddin A, Bhatia S (2005) Compar-ison of kinetic resolution between two racemic ibupro-fen enters in an enzymatic membrane reactor. ProcessBiochem 40(7):2417–2425


Enantioselective Transport of AminoAcids by Membrane Operations

Keiji SakakiNational Institute of Advanced Industrial Scienceand Technology (AIST), Research Institute forSustainable Chemistry, Tsukuba, Ibaraki, Japan

Amino acids are molecules that contain anamino group (–NH2), a carboxylic acid group(–COOH), and a side chain in each structure and

are the structural units of proteins. The most ofamino acids found in nature are categorized asa-amino acid in which the amino group is attachedto the carbon atom adjacent to the carboxylic acidgroup. An a-amino acid can exist in either of twooptical isomers except glycine, and the isomer isnormally described as L- or D-aminoacid. Themajor-ity of amino acids in living organisms are L-isomers.Optical isomers, including amino acids, frequentlyshow different bioactivity from each other. So, opti-cal resolution of amino acids has been an importantsubject in various industries dealing with pharma-ceuticals, foods, agrochemicals, and so on.

Optical resolution with membrane processes haspotential merits over other separation methods likecrystallization or chromatography.Membrane oper-ation can be carried out continuously and it enablesthe large-scale separation. Optical resolution ofaminoacids canbeachievedby the functionofchiralrecognition sites of solid membranes and liquidmembranes. The method using liquid membranesis described in another site, so solid membranes forenantioselective separation are explained here.A main research topic on enantioselective mem-branes is the design of the membranes that have thechiral recognition sites. Some enantioselective solidmembranes have been reported as follows.

Membranes Prepared from ChiralPolymers

Polypeptide and polysaccharide, like cellulose,alginate, and chitosan, and their derivatives arechiral polymers, and membranes from thesechiral polymers have been applied to theenantioselective separation of racemic aminoacids. The separation factor of these membranestoward racemic tryptophan is usually in the rangeof 1.1–5.0 (Higuchi et al. 2010). Membranes pre-pared from polymers with chiral branch have beenalso reported.

Membranes Added with Chiral Selectors

Enantioselective membranes can be prepared bythe addition of chiral selectors that show

702 Enantioselectivity

enantioselective affinity toward amino acids.Cyclodextrins, bovine serum albumin (BSA),and DNA have been used as chiral selectors. Themembranes added with these chiral selectorsshowed the separation factor of 1.1–2.7 towardamino acids (Higuchi et al. 2010).

Molecular Imprinting Membranes(MIMs)

Molecular imprinting membranes are fabricatedby incorporating the template molecules into themembranes and then removing the template mol-ecules. The formed cavities in the membranesshow the specific affinity with the template mole-cules and their analogue (Yoshikawa et al. 1995).Although MIMs currently show low enantios-electivity, this method has the potential of higherenantioselectivity (Xie et al. 2008).

Solid enantioselective membranes can bedivided into two classes: diffusion-selective mem-branes and sorption-selective membranes (van derEnt et al. 2001; Xie et al. 2008). A diffusion-selective membrane is defined as a membranewith no specific chiral selectors for the chiralinteraction but consists of a chiral polymer. Dif-fusion selectivity is caused by the chiral discrim-ination during diffusion. On the other hand, in asorption-selective membrane, chiral selectors areembedded in a polymer matrix. To reach perme-ation selectivity in sorption-selective membranes,the selectively adsorbed population has to bemobile. An electrical potential is sometimes usedfor the purpose.

References

Higuchi A, Tamai M, Ko Y, Tagawa Y, Wu Y, Freeman B,Bing J, Chang Y, Ling Q (2010) Polymeric membranesfor chiral separation of pharmaceuticals and chemicals.Polym Rev 50:113–143

van der Ent EM, van’t Riet K, Keurentjes JTF, van der PadtA (2001) Design criteria for dense permeation-selectivemembranes for enantiomer separations. J Membr Sci185:207–221

Xie R, Chu L, Deng J (2008) Membranes and membraneprocesses for chiral resolution. Chem Soc Rev37:1243–1263

Yoshikawa M, Izumi J, Kitano T (1995) Molecularlyimprinted polymeric membranes for optical resolution.J Membr Sci 108:171–175

Enantioselectivity


Enantioselectivity is the preferential formation ofa chemical reaction (mainly with metal complexescatalysts) of one enantiomer compound over theother (McNaught and Wilkinson 1997). Theextent of enantiomeric reactions is determinedby the percentage enantiomeric excess (% e.e)given by;

% e:e ¼ S� R

Sþ R� 100

References

McNaught AD,Wilkinson A (Eds.), (1997) IUPAC - Com-pendium of Chemical Terminology, Blackwell ScienceLtd., Oxford, England, UK

Enantiospecificity


Enantiospecificity is the ability of an enzymeto convert a racemic mixture into a particularenantiomer. The degree of specificity is interre-lated with that of selectivity, which equationsuggested by Lopez and coworkers (Lopezet al. 1990):

Eo ¼ E2 tanh F=E0:5� ��1 � E0:5=F

� �tanh Fð Þ�1 � F�1

" #2

Encapsulation 703

where

E ¼ ln 1� c 1þ eep� �

ln 1� c 1� eep� �

and

E

F ¼ vobsDeff So

R

3

� �2

and c is the conversion of the particularreaction (Tsai and Dordick 1996). If the calculatedintrinsic enantioselectivity is extremely high,therefore, the reaction can be considered asenantiospecific.

References

Lopez JL, Wald SA, Matson SL, Quinn JA(1990) Multiphase membrane reactors for separatingstereoisomers. Ann N Y Acad Sci 613(Enzyme Engi-neering 10):155–166

Tsai S-W, Dordick JS (1996) Extraordinary enantiospe-cificity of lipase catalysis in organic media induced bypurification and catalyst engineering. BiotechnolBioeng 52(2):296–300

Encapsulation

Goran T. Vladisavljević and Richard G. HoldichDepartment of Chemical Engineering,Loughborough University, Loughborough,Leicestershire, UK

Synonyms

Encapsulation: entrapping, enclosing, enveloping;Amphiphilic substances: Amphiphiles

Definitions

Encapsulation: Coating or entrapping of activeingredient inside a solid shell or within a liquidor solid matrix of another material.

Complex coacervation: Coacervation causedby the interaction of two oppositely chargedmacromolecules.

Internal phase separation: Phase separationwithin emulsion droplets consisted of a mixtureof polymer, good solvent and poor solvent, usu-ally triggered by removal of good solvent.

Micelles: Spherical aggregates of surfactantmolecules dispersed in an aqueous solution.

Liposomes: Spherical vesicles whose shellsconsist of single or multiple concentric bilayersresulting from the self-assembly of phospholipidsin an aqueous solution.

Colloidosomes: Shperical vesicles whose shellsconsist of coagulated or fused colloid particles.

Polymersomes: Shperical vesicles whoseshells consist of amphiphilic block copolymers.

Amphiphilic molecules: Surface active mole-cules made up of two parts, a polar or electricallycharged hydrophilic part and a hydrophobic part,most often an alkyl chain.

Polycondensation: A polymerization in whichthe growth of polymer chains proceeds by con-densation reactions between molecules of alldegrees of polymerization.

Layer-by-layer polyelectrolyte deposition: Athin film fabrication technique based on deposit-ing alternating layers of oppositely charged poly-electrolytes with wash steps in between.

Encapsulation is a process of enclosing orentrapping a core material (liquid, gas, solid par-ticles, cells, dissolved active ingredients, etc.)inside a solid shell or within a solid or liquidmatrix for the purpose of controlled or triggeredrelease, immobilization, isolation, or protection ofthe encapsulated material. The material beingencapsulated is called the core material, and thecarrier material used for envelopment or entrap-ment is called the shell material. Typical examplesof core materials are ink or dye for carbonlesscopy papers, liquid crystals for microparticle-based displays, phase-change material for smarttextiles, high-molecular-weight gases for ultra-sound contrast imaging, genetic material forin vitro compartmentalization, active food ingre-dients for functional food products, enzymes forbiocatalytic reactors, drugs, pesticides, fra-grances, antimicrobial agents, etc.

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Core/shella

c d

b

Multiple cores Multiple shells

Matrix type

Encapsulation, Fig. 1 Schematic diagrams of capsuleswith different morphologies

704 Encapsulation

Encapsulation efficiency (EE) is defined as thepercentage of core material incorporated into themicrocapsules relative to the total amount of thecore material added during encapsulation process:EE= (mE/mT) 100%, where mE is the mass of thecore material incorporated andmT is the total massof the core material added. Loading capacityrefers to the percentage of core material incorpo-rated within the microcapsules relative to the totalmass of the microcapsules (i.e., core + shellmaterial).

Two main types of capsules can be distin-guished, core/shell (reservoir) type and matrixtype. In the core/shell capsule (Fig. 1a), the shellmaterial completely surrounds and contains aninternal core material (the internal phase). Thestrategies used for formation of shell are spraycoating, complex coacervation, polymer precipi-tation by internal phase separation, interfacialreaction that may include polycondensation orcross-linking, and layer-by-layer polyelectrolytedeposition (Yow and Routh 2006). The core mate-rial can be released from core/shell capsules bysimple molecular diffusion through the shell or by

deliberately compromising the integrity of theshell, e.g., by exposing the microcapsule to envi-ronmental stresses such as mechanical forces,change of pH, temperature, or ionic strength, orby chemical or biochemical degradation of theshell. A special class of core/shell capsules aremicelles and vesicles (liposomes, polymersomes,and colloidosomes), formed by self-assembly ofamphiphilic molecules (phospholipids anddiblock copolymers) or particles (Dinsmoreet al. 2002). Micelles are formed spontaneouslywhen amphiphiles are dispersed in a polar solventat concentrations that exceed a critical level,known as the “critical micelle concentration”(CMC). Micelles containing solubilized materialsare referred to as microemulsions. A differencebetween micelles and vesicles is that vesicles arenot a thermodynamically stable state of amphi-philes and do not form spontaneously, e.g., with-out input of external energy (Tadros 1993).

In the matrix-type capsule (Fig. 1b) the corematerial is distributed uniformly throughout amatrix of shell material. The most commonrelease pattern from matrix-type capsules is firstorder in which the release rate decreases exponen-tially with time until the active ingredient isexhausted. Typical matrix-type capsules are mul-tiple emulsions, hydrogel particles, solid lipidparticles, polymeric particles, etc. Hybrid struc-tures consisting of a number of hierarchicallyassembled homogenous phases may beengineered that allow for even finer control overthe functionality of microcapsules (Fig. 1c, d).

References

Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M,Bausch AR, Weitz DA (2002) Colloidosomes: selec-tively permeable capsules composed of colloidal parti-cles. Science 298:1006–1009

Tadros TF (1993) Industrial applications of dispersions.Adv Colloid Interface Sci 46:1–47

Yow HN, Routh AF (2006) Formation of liquidcore–polymer shell microcapsules. Soft Matter2:940–949

Encapsulation Application 705

Encapsulation Application

Goran T. VladisavljevicDepartment of Chemical Engineering,Loughborough University, Loughborough,Leicestershire, UK

E
Synonyms
Bichromal particles synonym: Two-coloredparticles; Microencapsulation synonym:Microentrapment

Definitions

Carbonless Copy Paper - Paper coated on the backside with colorless dye or ink capsules, that burstunder the pressure of writing or typing and reactwith colorless clay particles coated on the uppersurface of the second sheet.; Phase Change Mate-rial - A substance with a high heat of fusion thatabsorb or release a high amount of energy duringmelting of freezing, thus acting as a thermalenergy storage; Electrophoretic display - a displaythat forms images by rearranging charged pigmentparticles with an applied electric field.

Microencapsulation can be done:

1. To protect the encapsulated material againstoxidation or deactivation due to reactionswith reactive species from the environment.

2. To mask the organoleptic properties like color,taste, and odor of the actives.

3. To achieve controlled/triggered/targetedrelease.

4. For safe handling of toxic materials.5. To achieve in vitro compartmentalization or

immobilization of biological materials andcatalysts.

Microencapsulated materials are utilized inagriculture, pharmaceuticals, foods, cosmetics

and fragrances, textiles, paper, paints, coatingsand adhesives, toner applications, and manyother industries.

Carbonless copy paper developed by Greenand Schleicher in the 1950s was the first commer-cial product to employ microcapsules. A coatingof microencapsulated colorless ink is applied tothe top sheet of the paper, and a developer isapplied to the subsequent sheet. When pressureis applied by writing, the capsules break and theink reacts with the developer to produce the darkcolor of the copy. Paper-like displays with lowpower consumption such as rotating bichromalmicrospheres system and microencapsulated elec-trophoretic system are other examples of commer-cial applications of microencapsulated ink(Yoshizawa 2004). In the rotating bichromal sys-tem (Gyricon), bichromal capsules with oppo-sitely charged hemispheres are free to rotatewithin oil-filled cavities. In the microencapsulatedelectrophoretic display system (E ink), oppositelycharged black and white particles move under anapplied electric field in a clear liquid encapsulatedwithin a transparent capsule.

Today’s textile industry makes use ofmicroencapsulated materials to enhance theproperties of finished goods. One applicationincreasingly utilized is the incorporation ofmicroencapsulated phase change materials(PCMs), such as paraffin wax. Phase changematerials absorb and release heat in response tochanges in environmental temperatures. Whentemperatures rise, the phase change materialmelts, absorbing excess heat, and feels cool. Con-versely, as temperatures fall, the PCM releasesheat as it solidifies, and feels warm. Microencap-sulation is also used in thermochromic and pho-tochromic fabrics, which change color withchanges in temperature or light, insect-repellentfabrics, which ward off mosquitoes, and scentedfabrics, which release fragrance (Nelson 2002).

Pesticides are encapsulated to be released overtime, allowing farmers to apply the pesticides lessoften rather than requiring very highly concen-trated and perhaps toxic initial applications

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706 Encapsulation Efficiency

followed by repeated applications to combat theloss of efficacy due to leaching, evaporation, anddegradation.

Ingredients in foods are encapsulated for sev-eral reasons (Gouin 2004). Most flavorings arevolatile; therefore, encapsulation of these compo-nents extends the shelf life of products byretaining the food flavors within that would oth-erwise evaporate out and be lost. Some ingredi-ents are encapsulated to mask taste, such asnutrients added to fortify a product withoutcompromising the product’s intended taste. Alter-natively, flavors are sometimes encapsulated tolast longer, as in chewing gum. Some food ingre-dients must be encapsulated to be protected fromoxidation or other degradation reactions causedby exposure to light, moisture, or oxygen. Micro-encapsulation preserves lactic acid bacteria, bothstarters and probiotics, in food and during thepassage through the gastrointestinal tract, andmay contribute to the development of new func-tional foods.

Many drug formulations for oral, intravenous,ocular, and subcutaneous administration aremicroencapsulated to achieve controlled,targeted, or triggered release of active ingredients.Aspirin, for example, can cause peptic ulcers andbleeding if doses are introduced all at once.

Microencapsulation of cells and enzymes isused to improve efficiency of bioreactors sincevery high volumetric productivity can beachieved; encapsulated biocatalysts typicallyhave greater thermal and operational stabilityand downstream processing is simplified, sincethe encapsulated biocatalyst can easily be recov-ered and reused. In molecular biology, single-cellencapsulation is used to achieve high-throughputscreening in directed evolution experiments.

References

Gouin S (2004) Microencapsulation: industrial appraisal ofexisting technologies and trends. Trends Food SciTechnol 15:330–347

Nelson G (2002) Application of microencapsulation intextiles. Int J Pharm 242:55–62

Yoshizawa H (2004) Trends in microencapsulationresearch. KONA 22:23–31

Encapsulation Efficiency


The encapsulation efficiency (EE%) is defined bythe concentration of the incorporated material(such as active ingredients, drugs, fragrances, pro-teins, pesticides, antimicrobial agents, etc.)detected in the formulation over the initial con-centration used to make the formulation.

Encapsulation efficiency (EE %) was calcu-lated using below formula:

EE % ¼ Wt=Wið Þ � 100%

where Wt is the total amount of the incorporatedmaterial and Wi is the total quantity of incorpo-rated material added initially during the prepara-tion. Wt and Wi can be determined usingspectroscopic or chromatographic method. If thecapsule shell material is a polymer, it can bedissolved in the solvent, and the incorporatedmolecule will get soluble and it can be quantified.If the incorporated molecule is not soluble in thatsolvent, it can be extracted by adding the capsulesin a liquid in which the target molecule is soluble(also bymultiple extraction). If the core material isa liquid (such as emulsions), the amount of theencapsulated material can be evaluated afterinduced separation of the liquid dispersed phaseand liquid continuous phase (i.e., simple emul-sions) or in the outer liquid phase (i.e., W2 inwater-in-oil-in-water (W1/O/W2) emulsions).The amount of water retained within the oil drop-lets during emulsification is also significant fordouble emulsion. The methods used in this partic-ular case enclose the measure of the outer waterphase conductivity by differential scanning calo-rimetry (DSC) (Schuch et al. 2013).

The encapsulation efficiency can be influencedby (i) the partition coefficient of the target mole-cule in the solvents used in the preparation of theformulation, (ii) the method used to carry out the

Encapsulation Techniques, Table 1 Classification ofdispersion techniques used in encapsulation processes

Liquid/air dispersion Liquid/liquid dispersion

Encapsulation Techniques 707

encapsulation process (temperature, pH, mechan-ical stress), and (iii) the size distribution of thecapsules (Jyothi et al. 2010).

Atomization Emulsification

Pressure nozzle High pressurehomogenizers

Two-fluid nozzle Ultrasound homogenizers

Spinning disc Static mixers

Dripping/jet breakup Rotor/stator devices

Simple dripping Microfluidic devices

Electrostaticextrusion

Membrane emulsification

Coaxial air/liquidflow

Microchannelemulsification

Jet cutting Inkjet printing

Centrifugal nozzle Micellization

E

References

Jyothi NVN, Prasanna PM, Sakarkar SN, Prabha KS,Ramaiah PS, Srawan GY (2010) Microencapsulationtechniques, factors influencing encapsulation effi-ciency. J Microencapsul 27:187–197

Schuch A, Köhler K, Schuchmann HP (2013) Differentialscanning calorimetry (DSC) in multiple W/O/W emul-sions, a method to characterize the stability of innerdroplets. J Therm Anal Calorim 111:1881–1890

Vibrating nozzle

Encapsulation Techniques, Table 2 Commonmethodsof solid shell/matrix formation in encapsulation processes

Mechanical/thermal Physicochemical Chemical

Cooling Solvent removal Suspension

Encapsulation Techniques

Goran T. VladisavljevicDepartment of Chemical Engineering,Loughborough University, Loughborough,Leicestershire, UK

polymerization

Freezing Evaporation ordrying

One stage(direct)

Two stage(droplet swelling)

Pan coating Liquidextraction

Interfacialpolycondensation

Fluidizedbed coating

Layer-by-layerdeposition

Sol-gel chemistry

Top spray Self-assembly

Bottomspray

Simple/complexcoacervation

Tangentialspray

Ionotropicgelation

Wursterprocess

Internal phaseseparation

The encapsulation technique of choice depends onthe type and physical properties of the core andshell material. The chosen encapsulation tech-nique should give a high encapsulation efficiencyand loading capacity of actives, capsules shouldnot exhibit aggregation or adherence, capsulesshould have a narrow particle size distributionwithout tails, threads, or dents on the surface,and the process should be suitable for industrialscale production.

Depending on the initial physical state of thecore phase, two different fabrication routes can bedistinguished: (i) coating solid particles by shell-forming material in a fluidized bed or pan coaterand (ii) dispersing a capsule-forming material inthe form of droplets in another immiscible liquidor air, followed by droplet solidification. There isa variety of atomization and dripping processes bywhich a liquid phase can be dispersed in anotherimmiscible fluid (Table 1).

Depending on the chemical composition of theshell-forming material, solidification of airsuspended droplets can be achieved by solvent

evaporation, cooling, or cross-linking in a hard-ening bath. Emulsification routes involve emulsi-fication of a solution or suspension of actives,followed by shell/matrix formation by internalgelation, polymerization, layer-by-layer electro-static deposition, internal phase separation, coac-ervation, etc. (Table 2).

Hydrogel capsules contain a hydrophilic activeentrapped within a hydrophilic polymer network

708 Encapsulation: Entrapping, Enclosing, Enveloping

that can absorb and hold large amount of water.A gel network can be formed by chemical gelation(polymerization by free radical processes or viacondensation) or by physical gelation, which caninvolve heating (heat-setting gels), cooling (cold-setting gels), or the addition of multivalent coun-terions (ionotropic gelation). In the internalionotropic gelation, the droplets of W/O emulsioncontain a gel-forming polymer (e.g., alginate) anda cross-linking agent in a nondissociated form(e.g., calcium carbonate), whereas the continuousoil phase contains a species (e.g., hydrogen ions)that diffuses into the droplets and triggers therelease of cross-linking agent in its active formand subsequent gelation. In the externalionotropic gelation, the droplets initially containonly a gel-forming polymer, and cross-linkingoccurs in a hardening bath (Zhang et al. 2007).

Coacervation involves the phase separation ofone or more polymers from the initial solution andthe subsequent deposition of the newly formedcoacervate phase around the active ingredientsuspended or emulsified in the same reactionmedia. In simple coacervation, phase separationis achieved by addition of desolvating agent(alcohol or salt) or by change in temperature orpH, whereas complex coacervation involves reac-tion between two oppositely charged polymers.The three basic steps in coacervation are(i) phase separation in a suspension or emulsionof active ingredient which leads to a three-phasesystem consisting of a polymer-rich liquidphase (coacervate phase), a polymer-lean liquid,and a solid or liquid phase containing activeingredient; (ii) deposition of the coacervatephase onto the dispersed particles or droplets;and (iii) hardening of the coating (Zuidam andShimoni 2010).

Solid lipid microparticles contain the activeingredient entrapped within a high melting pointlipid, such as fatty alcohols, fatty acids, fatty acidesters of glycerol, hydrogenated fatty acid esters,waxes, etc. Solid lipid microparticles can be fab-ricated by a temperature-controlled emulsificationof high melting point lipids followed by coolingor by emulsification/solvent evaporation method.In the latter case, a high melting point lipid isdissolved in an organic solvent, and the mixture

is emulsified with an aqueous phase at room tem-perature. The solid particles are then formed byevaporation of the organic solvent. Hydrophilicactives can be encapsulated by forming a W/O/Wemulsion prior to solvent evaporation or cooling(Jaspart et al. 2005).

Liposomes are usually prepared using twoapproaches: (i) hydration of dry lipid films withan aqueous solution of actives resulting in forma-tion of large multilamellar vesicles (MLV), whichis then followed by size reduction of MLVs bysonication, microfluidization, repetitive freezingand thawing, or extrusion through track-etchpolycarbonate membranes, and (ii) mixing anonaqueous lipid solution with an aqueous solu-tion (Walde and Ichikawa 2001).

References

Jaspart S, Piel G, Delattre L, Evrard B (2005) Solid lipidmicroparticles: formulation, preparation, characterisa-tion, drug release and applications. Expert Opin DrugDeliv 2:1–13

Walde P, Ichikawa S (2001) Enzymes inside lipid vesicles:preparation, reactivity and applications. Biomol Eng18:143–177

Zhang H, Tumarkin E, Sullan RMA, Walker GC,Kumacheva E (2007) Exploring microfluidic routes tomicrogels of biological polymers. Macromol RapidCommun 28:527–538

Zuidam NJ, Shimoni E (2010) Overview of microencap-sulates for use in food products or processes andmethods to make them. In: Zuidam NJ, Nedović VA(eds) Encapsulation technologies for active food ingre-dients and food processing. Springer, New York,pp 3–29

Encapsulation: Entrapping,Enclosing, Enveloping

▶Encapsulation

Engineered Organs

▶Bioartificial Organs, Membrane Operations of

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Enhanced Oil Recovery (EOR) 709

Engineered Organs and Tissue

▶Bioartificial Organs and Tissue

E

Enhanced Oil Recovery (EOR)

Arnaud BaudotPhysics and Analysis Division, IFP EnergiesNouvelles, Solaize, France

Enhanced oil recovery (EOR) aims at increasingthe oil field’s recovery yield, thanks to high-pressure injection of water or carbon dioxide.Injection of pressurized carbon dioxide leads tothe maintenance of high pressures in the reservoirand to an improvement of oil displacement,thanks to several physical effects: decrease of oilviscosity due to carbon dioxide solubilization,stripping effects, and modification of multiphaseequilibria due to CO2 solubilization in the reser-voir aqueous phase. When the oil reaches thesurface, carbon dioxide is purged with the associ-ated gas. Since the volumes of concerned carbondioxide are very large – from 140 to 280 Nm3 parextracted barrel (Mazur and Chan 1982) – it isnecessary to separate carbon dioxide from the

Enhanced Oil Recovery (EOR), Table 1 Comparison of(OPEX) of membrane-based, solvent-based, and hybrid proc

Operatingconditions

Study realizedby Year

CAPEXréf. Cryo

OPEXréf. + Me

175.000 m3/h Amoco(Goddin1982)

1982 47 M$

90 % CO2, 20.4 M$/a

18 bar

118.000 m3/h Permea(Boustanyet al. 1983)

1983 16.1 M$ 1.50

80 % CO2 6.5 M$/a 1.31

190.000 m3/h FluorEngineeringSchendel andSeymour1985)

1984

40 % CO2

aCryo.: cryogenic distillation (Ryan-Holmes process (Kohl anbHot potassium carbonate + membrane

hydrocarbon gaseous phase in order to have itrecycled to the reservoir (after pressurization).

Since the quantity of CO2 dissolved in oil variesover a large range of values during the wholeproduction of the reservoir (levels from 40 % upto 90 % can be observed after a few years ofproduction (Cooley and Dethloff 1985)), it is ratherdifficult to design a cost-effective CO2 removalunit that could operate during the entire life of thereservoir. Therefore, modular CO2 removal unitssuch asmembrane operations offer much flexibilityfor such an operation (Table 1).

The first large-scale EOR project operatingmembranes has been carried out in the Sacroc oilfield located in Western Texas. Carbon dioxideinjection was launched in 1972 (at a flowrange up to 240,000 Nm3/h). Three CO2 recoveryunits were initially installed: two hot potassiumcarbonate-based absorption units (operated bySun Explo with a 190,000 Nm3/h capacity leadingto a reduction of carbon dioxide from 24 % to0.5 % and Chevron, with a 54,000 Nm3/h lower-ing CO2 content down to 1 %) and an amine(MEA)-based absorption unit (operated byMonsanto, with a 20,000 Nm3/h capacity). Atthe end of the 1970s, realizing that the level ofCO2 was raising at a higher rate than expected,Chevron contacted Cynara in order to install andoperate two gas permeation units upstream of thehot potassium carbonate absorption columns. The

relative investment costs (CAPEX) and operating costsesses for carbon dioxide capture (EOR applications)

.a

Mem.

Mem.

DEA Cryo.a TEAKCO3

chaudm. +DEA

1 2.2 1.56 1.48

1 1.76 1.39 1.12

1 1.31

1 1.82

1 0.63 0.63b

1 0.56 0.68b

d Nielsen 1997))

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Enhanced oil recovery(Kvaerner Membrane Systems)

1 2 3 4 5 6

Flow rate (103 Nm3/h)Pressure (bar)

26.5 7.4 18.9 11.2 15.3 3.8

16 25 20 26 5 1

Composition (% mol.)

CO2 70 10 93.4 35.4 95.2 86.2

CH4 9.6 26.9 2.9 20 2.0 6.7

C2 6.3 10.0 1.0 14.0 0.7 2.4

C3 5.6 18.4 0.6 12.9 0.4 1.5

i-C4 2.5 8.6 - 6.0 - 0.2

n-C4 1.3 4.3 - 3.0 - 0.1

H2S 0.6 0.01 0.8 0.1 0.9 0.4

N2 4.1 11.8 1.1 8.6 0.7 2.5

CH4 (C2+) recovery yield = 77,8 % (73 %)

1

2

3

4

5 6

Enhanced Oil Recovery (EOR), Fig. 1 EOR example of permeation application (Spillman 1989)

710 Enhanced Oil Recovery (EOR)

treatment capacities of the membrane units were,respectively, 60,000 Nm3/h at 35 bar feed pressureprior to the Sun unit and 25,000 Nm3/h at 33 barfeed pressure prior to the Chevron unit. Cynarachose to operate a single-stage configuration, themembrane module being installed in parallel. Themembrane operation proved to be highly flexibleand reliable, as the membrane modules remainedin operation twice longer than the expected life-time (more than 5 years instead of 2–3 years)without any selectivity decrease. A slight decreasein membrane permeability has nevertheless beenobserved throughout the operation, and it wasnecessary to add more modules in order to main-tain the production level of the membrane opera-tion (Marquez and Hamaker 1986).

Since then, other membrane companies haveseen their products involved in other EORprojects:

– Kvaerner membrane systems (Chapelet al. 1999):Dallas Production Inc. (Texas) treated

200,000 Nm3 of gases containing up to25 % of carbon dioxide in 1994 during afeasibility study that lasted for 18 months.After this test, the membrane unit was inoperation treating 1200 Nm3/h.

Hydrocarbon Operating, Inc. (Texas)processed 590 Nm3/h at 51 bar feed pres-sure in 1994 on a pilot skid composed oftwo tubular membrane modules.

– Medal:A Medal membrane unit was operated at the

need of the 1990s to process 14,200 Nm3/hat 56 bar, 48 % CO2, reducing the retentateCO2 concentration down to 6 %.

– Cynara (Kohl and Nielsen 1995):Amoco, Texas (1994): Cynara membranes

were used to process 35,000 up to120,000 Nm3/h of gas containing 80% CO2.

Mobil, Salt Creek (1992): Cynara membraneswere used to process 76,000 up to120,000 Nm3/h of containing 70 % CO2.

Shell, Texas: Cynara membranes were used toprocess 13,000 Nm3/h of gas containing70 % CO2 (Fig. 1).

References

Boustany K, Narayan RS, Stookey DJ (1983) Economicsof removal of carbon dioxide from hydrocarbon gasmixtures. In: Abstracts 62nd Annual Gas ProcessorsAssociation, San Francisco, CA, pp 146–149

Chapel DG, Mariz CL, Ernest J (1999) Recovery of CO2

from flue gases: commercial trends. In: Oral Presenta-tion Canadian Society of Chemical Engineers AnnualMeeting, Saskatoon, Sakatchewan, Canada.

Cooley TE, Dethloff WL (1985) Field-tests show mem-brane processing attractive. Chem Eng Prog 81:45–50

Goddin CS (1982) Comparison of processes for treatinggases with high CO2 content. In: Abstracts 61st AnnualGas Processors Association Convention, Dallas, TX,pp 60–68

Kohl A, Nielsen R (1997) Gas purification, 5th edn. GulfPublishing Co, Houston, pp 1238–1295

Environmentally Friendly Solvents and Diluents 711

E

Marquez JJ, Hamaker RJ (1986) Development of mem-brane performances during SACROC operations. In:Oral presentation at AIChE Spring National Meetingand Petro. Expo’86. New Orleans, LA

Mazur WH, Chan MC (1982) Membranes for natural-gassweetening and CO2 enrichment. Chem Eng Prog78:38–43

Schendel RL, Seymour JD (1985) Take care in pickingmembranes to combine with other processes for CO2

removal. Oil Gas J 83:84–86Spillman RW (1989) Economics of gas separation mem-

branes. Chem Eng Prog 85:41–62

Environmentally Friendly Solventsand Diluents

Zhaoliang CuiState Key Laboratory of Materials-OrientedChemical Engineering, College of Chemistry andChemical Engineering, Nanjing Tech University,Nanjing, China

Non-solvent-induced phase separation (NIPS)and thermally induced phase separation (TIPS)are two major methods for polymeric porousmembrane preparation. Solvents and diluentsplay important roles in NIPS and TIPS processes,respectively, influencing the final membraneproperties during polymeric membrane formation.Applicable solvents for the NIPS process are N,N-dimethyl acetamide (DMAc), N,N-dimethylformamide (DMF), hexamethyl phosphoramide(HMPA), N-methyl pyrrolidone (NMP),tetramethylurea (TMU), triethyl phosphate(TEP), trimethyl phosphate (TMP), acetone(Ac), and tetrahydrofuran (THF), while applicablediluents for the TIPS process are dimethyl phthal-ate (DMP), diethyl phthalate (DEP), dibutylphthalate (DBP), dihexyl phthalate (DHP), ethylacetoacetate (EAA), propylene glycol carbonate(PGC), diphenyl ketone (DPK), diphenyl carbon-ate (DPC), cyclohexanone, camphor, andbutyrolactone. Most of the above solvents/dilu-ents are toxic, making the work atmospheredetrimental for workers and environment. Since

many membrane processes are employed withthe objective of environmentally improving theprocess industry, toxic solvents/diluents reducetheir contributions to environmental protection.Thus, finding more environmentally friendlysolvents/diluents is becoming an importanttopic in the membrane preparation field (Cuiet al. 2013a).

Recently, efforts have been done to developenvironmentally friendly solvents for NIPS pro-cess and diluents for TIPS process. These sol-vents/diluents should yield the membranes’promising properties and performances withoutdischarging toxicity to environment. It is muchbetter if the solvents/diluents can be recoveredeasily. Dimethyl sulfoxide (DMSO) and glycerintriacetate (GTA), which can be employed forNIPS and TIPS process, are relatively lesstoxic. They have been used for polymeric mem-brane preparation. Acetyl tributyl citrate (ATBC)(Cui et al. 2013b), which is more environmentallyfriendly, was introduced to prepare poly(vinyli-dene fluoride) (PVDF) membranes via TIPS pro-cess. Dimethyl sulfone (DMSO2) (Lianget al. 2013) was used as a universal crystallizablediluent to fabricate polar polymer membranes(PVDF, polyacrylonitrile (PAN) and celluloseacetate (CA)) via TIPS method. This diluent iseasy to recover because of its crystallizable prop-erty. The membranes fabricated by above diluentspresented promising mechanical properties andwater permeability. Ionic liquid is a type ofgreen solvents, which probably can be used forpolymeric membrane preparations. CA mem-branes (Xing et al. 2010) have been formed viaphase inversion employing 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN)as the solvent and were recycled. The morphol-ogy, porosity, and pure water permeability of theCA membranes fabricated by reused solventswere quite comparable to the CA membranesproduced by new solvents.

Researchers started to pay attention todevelop environmentally friendly solvents/dil-uents, and preliminary progress has beenmade. However, this is far from the require-ments, and more efforts are needed to contrib-ute to this field.

712 Environmentally Responsive Membranes

References

Cui Z, Drioli E, Lee YM (2013a) Recent progress influoropolymers for membranes. Prog Polym Sci39(1):164–198. doi:10.1016/j.progpolymsci.2013.07.008

Cui Z, Hassankiadeh NT, Lee SY, Lee JM, Woo KT,Sanguineti A, Arcella V, Lee YM, Drioli E (2013b)Poly (vinylidene fluoride) membrane preparation withan environmental diluent via thermally induced phaseseparation. J Membr Sci 444:223–236

Liang HQ, Wu QY, Wan LS, Huang XJ, Xu ZK(2013) Polar polymer membranes via thermallyinduced phase separation using a universal crystalliz-able diluent. J Membr Sci 446:482–491. doi:10.1016/j.memsci.2013.07.008

Xing DY, PengN, Chung TS (2010) Formation of celluloseacetate membranes via phase inversion using ionicliquid, [BMIM]SCN, as the solvent. Ind Eng ChemRes 49:8761–8769

Environmentally ResponsiveMembranes

▶Magnetically Responsive Membranes

Enzymatic (Peroxidase) MembraneBioreactor

María. T. Moreira1, Gemma Eibes2,Thelmo Lu-Chau2, Roberto Taboada-Puig2,Adriana Arca-Ramos2, Gumersindo Feijoo2,Juan M. Lema2 and Lucia Lloret31Department of Chemical Engineering, School ofEngineering, University of Santiago deCompostela, Santiago de Compostela, Spain2Department of Chemical Engineering, Instituteof Technology, University of Santiago deCompostela, Santiago de Compostela, Spain3Chemical and Environmental EngineeringDepartment, Federico Santa Maria TechnicalUniversity, Santiago, Chile

Enzymatic (peroxidase) membrane bioreactor isan enzymatic reactor coupled to a membrane in anumber of configurations that allows the

separation of the enzyme and its reuse in theenzymatic process.

This technology is especially suitable in con-tinuous operation by ensuring the recovery andreusability of the enzymes, allowing theminimization of large consumptions of biocata-lyst. Various enzymatic membrane bioreactorconfigurations are possible, i.e., enzymesuspended in solution in a reactor connectedwith a membrane unit, immobilized enzymewithin the membrane matrix itself, or entrappedenzyme in gel or microcapsules.

Among these setups, the retention of theenzyme by an ultrafiltration membrane is a veryinteresting alternative to overcome the washingout of the catalyst with the treated effluent butalso to avoid the enzyme immobilization whichis usually related with high costs, complex pro-cedures, and loss of enzyme catalytic activity. Inthis system the enzyme is added into a reactor tankwhich is coupled with an ultrafiltration membraneto enable the retention of the free enzyme and itsrecycling back to the reaction vessel. Thereby, byusing an enzymatic membrane reactor, it is possi-ble to separate the biocatalyst from productsand/or other substrates by a semipermeable mem-brane that creates a selective physical/chemicalbarrier (Fig. 1).

This type of bioreactor offers important bene-fits: high enzyme loads, prolonged enzyme activ-ity, high flow rates, reduced energy requirements,straightforward operation, and scale-up, and also,fresh enzyme can be easily added to maintainconstant enzymatic levels. Indeed, this systemhas been successfully operated for the continuousapplication of peroxidases (López et al. 2004) andlaccases (Lloret et al. 2013). Specifically,peroxidase-based membrane bioreactors wereapplied for the transformation of various com-pounds with bioremediation purposes, for exam-ple, for the removal of dyes (Orange II) andestrogenic compounds (estrone, estradiol,ethinylestradiol, etc.). Orange II was successfullytransformed by manganese peroxidase (MnP) inan enzymatic membrane bioreactor whichconsisted of a 250-mL stirred tank reactor(López et al. 2004 in Fig. 1) connected to apolyethersulfone synthetic membrane (Prep/

http://dx.doi.org/10.1007/978-3-662-44324-8_1431

Products

[1]

[2]MnPH2O2

Organic acidMn2+

Target compounds

MnP[Fe3+]

MnP-I[Fe4+=0 P·]

MnP-II[Fe4+=0]

H2O2

H2O

Mn2+ Mn3+

Mn3+

Mn2+

MnP-III

H2O2

H2O

Enzymatic (Peroxidase) Membrane Bioreactor, Fig. 1 Scheme of an enzymatic (peroxidase) membrane reactor

Enzymatic Membrane Reactor (EMR) 713

E

Scale-TFF Millipore) with a molecular weightcutoff of 10 kDa (Lloret et al. 2013 in Fig. 1).Three stock solutions: H2O2 (94–312 mM); Mn2+

(33 mM), Orange II (0.1 g/L), and organic acid(1 mM); andMnP (7,000 U/L, 125–225 U/L in thetank), were fed into the bioreactor by independentvariable speed peristaltic pumps, thus initiatingthe catalytic cycle of the enzyme. Under the bestconditions evaluated, high decolorization yieldcan be achieved and minimal enzymatic deactiva-tion, rendering an efficiency of 42.5 mg Orange IIoxidized per unit consumed of MnP.

The findings obtained with the proposed reac-tor configuration allow its application as a noveltreatment method for contaminated wastewaters.The simplicity of the operation, which mainlyrelies on the control of H2O2 dosage, promotesthe peroxidase membrane bioreactor for furtherscale-up applications. The oxidative potential ofthe MnP-H2O2-Mn2+ system appears to be gen-eral enough to be applied for the transformation ofa wide range of compounds. The main require-ment is that the oxidation potential of a particularsubstrate has to be lower than that provided by theenzymatic cycle, being the main factors affectingthe stoichiometry and kinetics of the process theMnP activity, dosage rate of H2O2, and concen-tration of the target compound.

References

Lloret L, Eibes G, Moreira MT, Feijoo G, Lema JM(2013) Removal of estrogenic compounds from filteredsecondary wastewater effluent in a continuous enzy-matic membrane reactor. Identification of transforma-tion products. Environ Sci Technol 47:4536–4543

López C, Moreira MT, Feijoo G, Lema JM (2004) Dyedecolourization by manganese peroxidase in an enzy-matic membrane reactor. Biotechnol Prog 20:74–81

Enzymatic Membrane Reactor (EMR)

Lidietta GiornoInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Synonyms

Enzyme membrane reactor

A general definition of an enzyme membranereactor (EMR) is a reactor system where a mem-brane separation is used to keep larger compo-nents in the reactor vessel (i.e., enzymes and/or

http://dx.doi.org/10.1007/978-3-662-44324-8_100106

714 Enzymatic Membrane Reactor (EMR)

macromolecular substrates), while low-molecular-weight molecules (i.e., productsand/or inhibitors) are allowed to pass freelythrough the membrane, thus leaving the reactoras permeate (Drioli and Giorno 1999). In thissetup, several advantages of immobilized prepa-rations, together with easy recovery of deactivatedenzymes and replacement with fresh catalysts areachieved; inhibitors are continuously removedfrom the reaction vessel. The direct and deepcontact between substrates and biocatalysts limitsdiffusional resistances, while no activity lossesdue to fixation to the support occur, thus maxi-mizing the activity of the biocatalyst. On the otherhand, enzyme stability is not improved.

Both dead-end and continuous stirred tankreactor (CSTR) ultrafiltration cells with flat mem-branes can be used as enzymatic reactors.

Performance of dead-end units is largelyaffected by the flow dynamics of the system; infact, mixing of substrates and catalysts is not fullyaccomplished, and concentration polarizationphenomena strongly limit reactor performance.

Continuous stirred tank reactors have beenmore widely adopted, both due to the possibilityof concentration polarization control and easymodeling of enzyme kinetic behavior.

Assuming complete mixing within the reactorso that enzyme and substrate concentration in thereactor vessel are uniform, and the latter is equalto its value in the permeate, substrate mass bal-ance in molar form can be written as:

Sf � Sð Þ=t ¼ R S, Pð Þ (1)

where

Sf = feed substrate concentration, ML�3

S = substrate concentration, ML�3

t = reactor time constant, TR = reaction rate, MT�1 L�3

Product steady state mass balance similarlywill be:

P� Pfð Þ=t ¼ R S, Pð Þ (2)

where

P = product concentration, ML�3

Pf = feed product concentration, ML�3

Assuming that substrate conversion obeys thesimple Michaelis-Menten model, substrate steadystate mass balance reduces to:

XSf þ X= 1� Xð Þ½ K0M ¼ K E V=Q (3)

R being expressed as K E S= K0M þ S

� �.

X ¼ Sf � Sð Þ=Sf = conversion degreeKM

0 = enzyme Michaelis-Menten constant, ML�3

K = enzyme kinetic constant, T�1

E = enzyme concentration, ML�3

V = reaction volume, L3

Q = flow rate, L3 T�1

The equation in this form is a useful tool toestimate parameters of reaction kinetics. Insteadof performing nonlinear parameters estimationprocedures, the functional dependence of XSf onX= 1� Xð Þ can be plotted. The plot should looklike a stright line whose slope is �K0

M

� �and

whose intersection with the XSf axis is given bythe point of coordinate (Vmaxt).

Enzyme activity is usually not constant withtime. Physicochemical changes in enzyme struc-ture, thermal denaturation, and microbial contami-nation cause enzyme activity to continuouslydecrease with time. When enzymes or cells arecompartmentalized in UF cells, biocatalystlosses can even occur due to the wrong choice ofmembrane molecular weight cutoff. It is conven-tional to measure the enzyme stability in termsof its half-life time (t1/2), that is, the time atwhich enzyme activity is reduced to half its initialvalue. It can be calculated from the followingequations:

Kd ¼ 2:303

#log

AE0

AE#

t1=2 ¼0:693

Kd(4)

Kd = enzyme deactivation constant, T�1

# = operation time, TAE0

= initial enzyme activity, or product mass perunit time and reaction volume, M T�1

AE#= enzyme activity at time #, M T�1

Enzymatic Membrane Reactor (EMR) 715

Since biotransformations bymeans of enzymesare continuous processes, as long as the reactorworking life is longer than the native enzyme half-life, enzyme activity decay with time must betaken into account in order to correctly assessreactor performance. A transient substrate massbalance on the CST reactor leads to the equation

E

V dS tð Þ=dt½ ¼ Q Sf � S tð Þ½ � VmaxV (5)

where

V = reaction volume, L3

S = substrate concentration, ML�3

Sf = feed substrate concentration, ML�3

Vmax = enzyme reaction rate at saturating sub-strate concentration, ML�3 T�1

Q = flow rate, L3 T�1

Let us assume that substrate inlet concentrationis much higher than the enzyme’s apparentMichaelis constant, KM

0 so that enzyme kineticsis expressed according to zero order kinetics, thatis, R ¼ KE ¼ Vmax . Enzyme activity decay canbe expressed in terms of the Arrhenius equation:

E ¼ Eo exp �Kdtð Þ (6)

where


E0 = initial enzyme concentration, ML�3

Kd = enzyme deactivation constant, T�1

Equation 5 then takes the form

Sf � S tð Þ½ =t� dS tð Þ=dt¼ Vmaxo exp �Kdtð Þ (7)

Integration of the differential Equation 7 leadsto estimate the outlet substrate concentration:

S tð Þ ¼ Vmaxo exp � t=tð Þ � exp �Kdtð Þ½ =1=t� Kdð Þ þ Sf

(8)

Estimation of the deactivation constant can begraphically carried out from Equation 7 plotting

ln Sf � S tð Þ½ =t� dS tð Þ=dtf g vs t. A straight lineis thus obtained whose slope is given by �Kdð Þand the line intersects the vertical axis at the pointof coordinate (ln Vmax).

When the reacting solution is fed to the system,low-molecular-weight products leave the reactorpermeating the membrane, whereas enzymes arepartially or totally rejected. Then, enzymes tendto accumulate in a thin layer immediatelyupstream from the membrane causing polarizationphenomena to occur. The extent to which concen-tration polarization affects reactor performancedepends on the balance of rejected solutes, e.g.,enzymes or cells, accumulation due to membranerejection and back diffusion to the bulk phase, andeventually on the flow dynamics of the reactingvessel.

For macromolecules (like most biocatalysts), ifmembrane properties are carefully chosen, mem-brane rejection is usually very good, while biocat-alyst back diffusion towards the bulk phase isextremely slow. The effect of concentration polar-ization on such reactor performances can then besignificant.

Applying the thin film theory to a regionimmediately upstream from the membrane resultsin the following steady state mass balanceequation:

JE ¼ �DedE=dx (9)

under the boundary condition (BC)x ! 1 E ¼ Es

where

J = volumetric flux, L3L�2T�1


ES = enzyme concentration in the bulk liquidphase, ML�3

De = enzyme diffusion coefficient, L2T�1

Under the assumption of totally rejectedenzyme macromolecules, integration of Equa-tion 9 allows one to express the ratio of enzymeconcentration in the bulk solution in the presenceof concentration polarization to its value inthe absence of concentration polarization phe-nomena as:

716 Enzymatic Membrane Reactor (EMR)

ES=Eo ¼ 1= 1� að Þ þ a=bð Þ exp bð Þ � 1½ f g(10)

Enzyme concentration at themembrane–solutioninterface can thus be related to enzyme concentra-tion in the bulk phase by the following equation:

Ew ¼ ES exp bð Þ (11)

where the dimensionless parameter b is a measureof convective mass transfer, J, relative to the over-all mass transfer, D/d, more generally KS.

The change of enzyme bulk concentration canbe dramatic, with changes in permeate flow rate,and applied pressure. Changes in enzyme bulkconcentration may induce a large reduction inreaction rate within the membrane reactor. Undergiven stirring conditions, therefore, a criticalvalue of flow rate and hence of applied pressuredoes exist. At flow rates lower than the criticalvalue, the reactor always attains steady operationconditions, and correspondingly outlet productconcentration is constant. Beyond the criticalvalue, concentration polarization phenomena pro-mote the localization of a large fraction ofenzymes near the membrane surface, seldom lead-ing to an enzymatic gel formation. Correspond-ingly, an accelerated deactivation of enzymeactivity is superimposed on reactor performancethus hindering the attainment of steady state con-ditions. Experimental evidence suggests that thecontribution of polarized enzymes to overall con-version can be negligible, due to the consistencyof product concentration in the bulk phase of thereactor and in the permeate.

When macromolecular substrates are involvedin the transformation under study, concentrationpolarization phenomena affect the EMR perfor-mance more severely. Diffusion limitations ofmacromolecular substrates hamper the useof immobilized enzymes in the hydrolysis ofhigh-molecular-weight substrates. By selectingmembranes with an appropriate molecular weightcutoff, both enzyme and substrate are retained inan EMR in touch with each other, and hydrolysis

products and/or inhibitors are continuouslyremoved from the system. Soluble enzymes canthen act directly on substrate macromoleculeswithout diffusion limitations and steric hindranceimposed by enzyme fixation to a solid support.The stirring features of CST EMRs moreoverassures that substrates and/or inhibitors withinthe reactor vessel are maintained at the lowestpossible concentration level. Such reactor config-uration is then extremely useful when substrateinhibited reaction patterns are involved or wheninhibiting species are assumed to exist in the feedstream.

Diafiltration, semicontinuous, and continuousoperational modes can be used.

Operating the reactor in a semicontinuous con-dition and adding substrate so as to keep its bulkconcentration constant leads to meaningfulchanges in reactor performance as compared tothe diafiltration mode. Under both operationalmode, permeate flow rate continuously decreaseswith time.

When EMRs are operated continuously, feed-ing a slurry of substrate macromolecules to thereactor, concentration polarization phenomenaplay a dominant role. The presence within thereaction vessel of contaminants or intermediateproducts, which are not fully hydrolyzed by theenzymatic system under study can lead to mem-brane fouling or to the formation of a gel layer atthe membrane surface. Under such conditions, thefiltration rate continuously decreases with time,and it may happen that substrate conversion doesnot attain steady state conditions. The addition ofenzymes capable of hydrolyzing such foulants tolow-molecular-weight compounds usuallyimproves reactor performance, eventuallyapproaching steady state conditions in terms ofboth permeate flow rate and substrate conversion.In addition, antifouling procedures including suit-able feed pretreatment or procedures to reduceconcentration polarization can be used.

Equation 11 demonstrates the dependence ofenzyme concentration at the upstream membranesurface on the flow dynamics of the reaction ves-sel. Due to the monotonicity of the exponential

Enzymatic Membrane Reactor in Supercritical CO2 717

E

function, at given applied pressure operationalconditions improving mass transfer in the bulkphase (i.e., increasing axial flow rate improvesKS or D/d) diminish concentration polarization.High stirring speeds usually improve filtering per-formance of a CST flat membrane reactor, albeit atthe expense of partial enzyme deactivation.

Reactor configurations other than dead end orCSTR/UF cell can offer improvements.

The use of hollow fiber ultrafiltration modulesin cross-flow filtration mode strongly improvesboth reactor performance and economics. Capil-lary membranes are characterized by a favorablesurface to volume ratio; advantages from this fea-ture are not only related to lower overall plant sizebut also to the increase of the surface to price ratio.Flushing enzyme/substrate solution through UFmodule tangentially to the membrane surface ata high linear velocity reduces the extent of con-centration polarization avoiding the formation of asecondary enzymatic gel membrane. Providedthat the volume of the UF module is small relativeto the total volume, and that recirculation flow rateis much larger than permeate flux, system kineticbehavior can be modeled in terms of a CSTreactor.

References

Drioli E, Giorno L (1999) Biocatalytic membrane reactors:application in biotechnology and the pharmaceuticalindustry. Taylor & Francis, London

Enzymatic Membrane Reactorin Supercritical CO2

Gilbert M. RiosPlace Eugene Bataillon, IEM – EuropeanMembrane Institute, Montpellier, France

Enzymes are catalysts obtained from nature(proteins), which work in mild conditions and

present very specific activities. They are one ofthe main tools of life as it developed since theorigin. Enzymes are divided into six classes,based on the type of reaction that is catalyzed.Lipases are important enzymes that belong to theclass of hydrolases; they catalyze hydrolytic reac-tions. Candida antarctica lipase B (CalB) is oneof the most widely used lipases, because thisenzyme accepts a broad spectrum of different sub-strates. Most of the time enzymes are immobilizedfor industrial applications. The main reason toimmobilize enzymes lies in the reduction ofcosts by enabling efficient separation, recycling,and reuse of the expensive enzymes. The choicefor the best immobilization method depends onthe enzyme, the type of reaction, and the reactionenvironment.

The use of porous membrane reactor for enzy-matic catalysis represents from this viewpoint abreakthrough technology which offers manyadvantages as compared to other more classicalsystems (immobilization on divided solid parti-cles in fixed, fluidized, or moving beds): continu-ous mode, reduction of product/substrateinhibition, free enzyme end-products, and the pos-sibility to set up monophasic or biphasicsystems – even if there can be some drawbackrelated to membrane fouling. The full activity maybe maintained inside the reactor by the rejection ofenzyme molecules (Fig. 1) either freely circulat-ing (a) or immobilized on/in the membrane (b). Inthat case, the membrane is the catalyst media(functionalization) and the reaction occurs duringmembrane crossing. The convective control dueto the application of a low transmembrane pres-sure (very small thickness of the porous layer: afew microns) is responsible for the mass transferincrease. The membrane is a macrosystemresulting from the assembly of swarms ofmicrosystems: each pore represents a particularmicroreactor.

The main practical use of enzymatic catalysisin supercritical carbon dioxide (scCO2) has to befound in producing high-added-value productssince the process remains expensive. Lipaseshave proven to be rather stable and active during

E

S

P P

a b

E

E

S P

membranemembrane DiffusionP

PSa b S

convection

Enzymatic Membrane Reactor in Supercritical CO2, Fig. 1 Basic mechanisms

Viscousliquid SCF

injectionchamber

Membraneseparation

Viscous liquid + Dissolved SCFPermeate :

Retentate(recycled)

Viscous liquid

SCF

Fluidification process

Pure SCF Extraction chamber

Membrane separation

SCF + Dispersed moleculesPermeate :

Retentate(recycled)

Molecules

SCF

SCFE + NF process

XCO2

Pressure

1.0

20 MPa

XCO2

Pressure

0.2

20 MPa

Enzymatic Membrane Reactor in Supercritical CO2, Fig. 2 Plant schemas

718 Enzymatic Membrane Reactor in Supercritical CO2

long-term reactions in scCO2. However, it cannotbe stated that performing reactions in scCO2 willalways result in a higher activity. This is highlydependent on the type of reaction, the enzyme,and the applied conditions. Themain advantage ofCO2, a very stable and nontoxic fluid, is therefore

the possibility of easier separation steps with com-plete removal of the solvent (Fig. 2).

The future perspectives for enzymatic mem-brane reactors in scCO2 can be found in thecombination of improving the stability andactivity of the potentially interesting

Enzymatic Ultrafiltration Membranes (Immobilization of Urease and Trypsin) 719

E

enzymes – other types of enzymes are attractivefor use in scCO2 as well, such as oxygenases,epoxide hydrolases, aldolases, or decarbo-xylases – and an improved integration of thereaction and separation sections. Very interest-ing studies conducted lately on transesteri-fication reactions with the fixed enzymeconfiguration (Fig. 1b), either using pure SCfluid solvent containing dissolved substrates orfor transforming oils after the injection of asmall quantity of pressurized CO2 in order todecrease viscosity, are worth mentioning.

References

Dijkstra ZJ (2006) The potential of enzymatic catalysis insupercritical fluids. PhD thesis, Eindhoven Universityof Technology (NL). ISBN-10: 90-386-2698-3/ISBN-13: 978-90-386-2698-7

Pommier E, Delebecque N, Paolucci-Jeanjean D, Pina M,Sarrade S, Rios GM (2007) Effect of working condi-tions on vegetable oil transformation in an enzymaticreactor combining membrane and supercritical CO2.J Supercritl Fluids 41(3):380–385

Rios GM (2009) Enzymatic membrane reactor. Plenary lec-ture. PERMEA, Prague. 9–11 June 2009. http://www.imc.cas.cz/sympo/permea09/programme_plenary.html

Enzymatic Ultrafiltration Membranes (Immobilization oadsorption of polyelectrolytes on membrane

Enzymatic Ultrafiltration Membranes(Immobilization of Ureaseand Trypsin)

Christophe InnocentInstitut Européen des Membranes, ENSCM,CNRS, Université de Montpellier, Montpellier,France

The sodium alginate was used as an anionic poly-electrolyte. The polyethyleneimine was used as acationic polyelectrolyte. Trypsin (enzyme whichcatalyzed the hydrolysis of peptide bonds inwhich the carboxyl groups are contributed by thelysine and arginine residues) was applied as apositively charged polyelectrolyte and the urease(enzyme which catalyzes the hydrolysis of urea toammonium and carbon dioxide) as a negativelycharged polyelectrolyte. For adsorption of thepolyelectrolyte layers, the supporting membrane(polyacrylonitrile-modified membrane negativelycharged) was immersed in the solution of thecationic polyelectrolyte, rinsed with the buffersolution to remove excess of polycations on

f Urease and Trypsin), Fig. 1 Scheme of layer-by-layer

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720 Enzyme Compartmentalization

membrane surface, immersed in the solution ofthe anionic polyelectrolyte, and rinsed with thebuffer solution again. These steps were repeatedto form multiple polyelectrolyte bilayers based onthe electrostatic layer-by-layer self-assembly(Fig. 1).

References

Decher G (1997) Fuzzy nanoassemblies: toward layeredpolymeric multicomposites. Science 277:1232–1237

Enzyme Compartmentalization

Rosalinda MazzeiInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Synonyms

Immobilization of enzymes

In biology enzyme compartmentalization is effi-ciently carried out by biomembrane or biologicalmembrane that permits to confine specific func-tions given by enzymes in a precise space. Theenzymes can be localized inside the delimitedcompartments or on the membrane surface.

In eukaryotic cells the corresponding delimitedspaces or compartments are called organelles,from which the main are the endoplasmic reticu-lum, Golgi apparatus, nucleus, mitochondria,lysosomes, endosomes, and peroxisomes. Eachmembrane-enclosed organelle contains a specificset of proteins free or on the membrane surfacethat regulates many vital biochemical processes.For instance, lipid metabolism is catalyzed mostlyby membrane-bound enzymes.

Starting from nature simulation, in biotechnol-ogy process, enzyme compartmentalization canbe obtained by immobilizing proteins on supportsor delimiting them by support compartments.Membranes can be optimal supports for enzymecompartmentalization; in fact they are widely

used for the production of membrane bioreactorsor biocatalytic membrane reactors (Mazzeiet al. 2010). In particular, asymmetric hollowfiber membrane can be used as efficient supportto compartmentalize enzymes by entrapment intothe sponge membrane layer. The general mem-brane requirement is that the pores in the denselayer are small enough to retain enzyme mole-cules and at the same time large enough to freelypass substrates and products (Mazzei et al. 2013).

References

Mazzei R, Drioli E, Giorno L (2010) Biocatalytic mem-branes and membrane bioreactors. In: Drioli E andGiorno L (eds) Comprehensive membrane science andengineering, vol 3. Elsevier B.V., Oxford, pp 195–212

Mazzei R, Piacentini E, Drioli E, Giorno L (2013) Mem-brane bioreactors for green processing. In: Boodhoo K,Harvey A (eds) Process intensification for green chem-istry: engineering solutions for sustainable chemicalprocessing. John Wiley & Sons, Ltd., Chichester,pp 227–250

Enzyme Membrane Reactor

▶Enzymatic Membrane Reactor (EMR)

Enzyme Membrane Reactors

▶Membrane Bioreactors

Enzymes Immobilized on Lumen

Rosalinda MazzeiInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Synonyms

Immobilization of enzymes

http://dx.doi.org/10.1007/978-3-662-44324-8_100271

http://dx.doi.org/10.1007/978-3-662-44324-8_1493

http://dx.doi.org/10.1007/978-3-662-44324-8_2242

http://dx.doi.org/10.1007/978-3-662-44324-8_100271

Enzymes Immobilized on Lumen, Fig. 1 Scheme oftubular biocatalytic membrane with biocatalystimmobilized on lumen surface

Ethanol Production by Continuous Fermentation-Pervaporation 721

E

This particular kind of immobilization is generallyreferred to enzymes immobilized into the mem-brane lumen surface of tubular membranes for theproduction of biocatalytic membrane and biocat-alytic membrane reactors. A scheme of a crosssection of a biocatalytic asymmetric hollow fibermembrane is reported in Fig. 1, in which the placethat the biocatalyst was immobilized on the mem-brane lumen surface is highlighted.

The biocatalyst can be also compartmental-ized/segregated in the lumen of tubular membraneso confined in a defined region of the membranemodule space. In these reactors, the enzymes orcells are not linked to the membrane. The mem-brane in this particular case can retain the enzymeor the cell which is not lost in the effluent stream,while low-molecular-weight products and inhibi-tors can be removed through the membrane(Strathmann et al. 2006).

References

Strathmann H, Giorno L, Drioli E (2006) An introductionto membrane science and technology. ConsiglioNazionale delle Ricerche, Roma

EOD


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Ethanol Production by ContinuousFermentation-Pervaporation

Wanqin JinState Key Laboratory of Materials-OrientedChemical Engineering, Nanjing University ofTechnology, Nanjing, China

Ethanol production by continuous fermentation-pervaporation is a process that couples ethanolfermentation with in situ pervaporation(PV) recovery of ethanol from fermentation broth.

With the fast increasing of fossil fuel consump-tion and the deterioration of environment, it iscommonly accepted that the fossil fuel forhuman energy use has to be replaced generallyby renewable resources. Compared with tradi-tional resources, bioethanol produced by fermen-tation process can be sustainably developedwithout extra yield of greenhouse gas (Penget al. 2011). Bioethanol is produced from renew-able resources (biomass) by ethanol fermentationprocess. However, usually, the maximum concen-tration of solvents (ethanol) does not exceed8 wt% owing to the end-product inhibition onmicroorganism, resulting in high energy cost toseparate ethanol from the dilute fermentationbroth by distillation. Therefore, several in situproduct-removal technologies, such as adsorp-tion, gas stripping, liquid-liquid extraction,perstraction, pervaporation, and reverse osmosis,have been developed. These technologies couldbe coupled with ethanol fermentation process todecrease the effect of product inhibition andimprove the sugar utilization and solvent produc-tivity (Vane 2005).

Pervaporation coupled with fermentation,which is also called fermentation-pervaporationcoupled process, has been regarded as the mostpromising separation method because this process

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Ethanol Production by Continuous Fermentation-Pervaporation, Fig. 1 Ethanol fermentation-pervaporationcoupled process

722 Ethanol Production by Continuous Fermentation-Pervaporation

can in situ extract organic components which isharmless for microorganisms in mild condition.Figure 1 shows its typical flow chart (Vane 2005).A pervaporation membrane module is connectedto the fermentor to form a circulation of fermen-tation broth. Preheated fermentation solution iscontinuously circulated from fermentor throughthe membrane upstream side with another sidebeing vacuumed using vacuum pump. Therefore,ethanol solvent can be selectively and continu-ously removed from the fermentation broth andthen concentrated in the membrane downstreamside. As a result, ethanol concentration in thebroth is kept at low level to facilitate the continu-ous fermentation process as well as enhance thesolvent productivity. In addition, a microfiltration/ultrafiltration unit can be alternatively installedbefore the pervaporation module, in order to filtermicrobes and avoid the biofouling ofpervaporation membranes.

PV membranes can be made from either poly-meric or inorganic materials, even both of them(Peng et al. 2011). The polymeric pervaporationmembranes for extracting ethanol from fermenta-tion broth include polydimethylsiloxane (PDMS)membranes, poly[1-(trimethylsilyl)-1-propyne]

(PTMSP) membranes, poly(ether block amide)(PEBA) membranes, liquid membranes, othermodified polymer membranes, as well as porouspolypropylene (PP) membranes and polytetra-fluoroethylene (PTFE) membranes. Amongthem, the commonly used PDMS membraneswith good selectivity and stability are regardedas the most potential PV membranes for ethanolrecovery application (Xiangli et al. 2008). Thetypical inorganic membrane for PV is hydropho-bic zeolite membrane, such as ZSM-5 andSilicalite-1 membrane.

References

Peng P, Shi B, Lan Y (2011) A review of membranematerials for ethanol recovery by pervaporation. SepSci Technol 46:234–246

Vane LM (2005) A review of pervaporation for productrecovery from biomass fermentation processes. J ChemTechnol Biotechnol 80:603–629

Xiangli FJ, WeiW, Chen YW, JinWQ, XuNP (2008) Opti-mization of preparation conditions for polydimethyl-siloxane (PDMS)/ceramic composite pervaporationmembranes using response surface methodology.J Membr Sci 311:23–33

Ethanol–Water Mixtures: Separation by Pervaporation 723

Ethanol–Water Mixtures: Separationby Pervaporation

Kew-Ho LeeCenter for Membranes, Korea Research Instituteof Chemical Technology (KRICT), Yuseong-gu,Daejeon, South Korea

E
Pervaporation is an important membrane processin chemical industries in which valuables are iso-lated from the liquid mixture. Liquid and vaporseparation by thermal processes has always beenhighly energy intensive, and new separation pro-cesses taking advantage of mass transfer throughdense membranes have already shown they enablevery significant energy savings as compared tomore classic technologies (Anne et al. 2002).Membranes can be used for the selective removalof water from aqueous organic mixtures.Pervaporation (PV) is a separation process thatinvolves separation of liquid mixtures, in contactwith a membrane. With feed solution on one side,permeate is removed as a vapor from the otherside (Brian et al. 2011); pervaporation (PV) is avery well-known membrane process for the sepa-ration of liquid and vapor mixtures due to itsenergetic aspects (EP 909209A1 1999; EP944575A1 1999; EP 880400A1 1998).
Ethanol–Water Mixtures:Separation byPervaporation,Fig. 1 Solution-diffusionmechanism (Graham 1866)

Pervaporation mostly allows a variety of possibleapplication areas: dewatering of organic fluidslike alcohols, ketones, ethers, etc. (EP 765682A11997); separation of mixtures from narrow boilingtemperatures to constant (azeotrope) boiling tem-peratures (EP 811420A1 1997); removal oforganic pollutants from water and air streams(EP 749351A1 1996); separation of fermentationproducts; and separation of organic-organic liquidmixtures (Kujawski 2000). Pervaporation is alsoconsidered as so-called clean technologies, espe-cially well suited for the treatment and recyclingof volatile organic compounds and pollution pre-vention (Anne et al. 2002).

Transport mechanism of PV through poly-meric membranes was studied by many researchgroups, and it was explained by the solution-diffusion model (Binning et al. 1961; Paul andPaciotti 1975; Lee 1975; Mulder and Smolders1984; Kataoka et al. 1991a, b). According to thesolution-diffusion model, each component of thepermeation molecules dissolves into the mem-brane and diffuses through the membrane due tothe concentration gradient (Mikihiro et al. 1998)(Fig. 1).

Transport through the membrane is driven bythe vapor pressure difference between the feedsolution and the permeate vapor. The vapor pres-sure difference can be maintained by applying avacuum on the permeate side or by cooling the

1. Sorption

Micrivoids

Microchannels

Polymer

2. Diffusion

3. Desorption

Ethanol–Water Mixtures: Separation by Pervaporation, Table 1 PV dehydration of ethanol through variouspolymeric membranes (Brian et al. 2011)

Polymer Feed (wt% water) Temp. (�C) Separation factor Flux (g/m2h)

Regenerated cellulose 50 45 5.0 2060

Cellulose acetate 4 60 5.9 200

Teflon-g-polyvinylpyrrolidone 4 25 2.9 2200

Perfluorinated polymer on PAN support 1.3 50 387 1650

Nafion-H+ 4 70 2.5 5000

Polyacrylonitrile-polyvinylpyrrolidone 4 20 3.2 2200

Poly(maleimide-co-acrylonitrile) 15 15 33 8

Poly(acrylic acid-co-acrylonitrile) 18 15 877 13

Polystyrene 4 40 101 5

Poly(vinyl chloride) 4 40 63 3

Alginic acid 4 40 8.8 48

5 60 13 2800

Chitosan 4 40 2208 4

Chitosan acetate salt 4 40 2556 2

Chitosan/glutaraldehyde 4 40 202 7

PVA/25 % TEOS, annealed at 160 �C 15 40 329 5

PVA/25 % TEOS, annealed at 130 �C 15 40 893 4

724 Ethanol–Water Mixtures: Separation by Pervaporation

permeate vapor so that it condenses, thus creatinga partial vacuum. Commercial systems for thedehydration of concentrated alcohol and othersolutions have been developed since the 1980s,much of the push coming from interest in theproduction of pure ethanol as an alternative liquidfuel, where PV can be used to dehydrate (Brianet al. 2011).

Pervaporation Membranes

Polymeric MembranesFor dehydration, where the small molar volumefavors the preferential sorption of water, materialshave to be selected with a higher affinity for waterthan for the other component. The polymericmaterials can be broadly classified into three cat-egories: glassy polymers, rubbery or elastomericpolymers, and ionic polymers. In general, theglassy and ionic polymers are more suited formaking water-selective membranes fordehydration.

For water-selective membranes, the mostimportant factor responsible for the separation isthe specific interaction between water and thepolymer. To obtain high selectivities, it is

necessary to use polymers, which contain specificgroups/active centers, capable of strong interac-tions with water.

The highest fluxes are those for the hydrophilicmembranes based on cellulose and Nafion andgrafts of hydrophilic poly(vinylpyrrolidone) onTeflon and polyacrylonitrile. The PVA/TEOSmembranes are exceptions in that they are hydro-philic but exhibit low fluxes (Brian et al. 2011)(Table 1).

The emphasis is on selectivity, postulated to bedetermined by selective sorption and selectivediffusion. Selective sorption is governed by thepresence in the membrane of active centers suchas charged sites which are capable of specificinteraction with water, while selective diffusionis governed by the rigidity and regularity of thepolymer structure and the nature of the polymerinterspace, exemplified by the degree of swellingand the frequency of the cross-links. The resultsfor a series of membranes made by grafting neu-tral or charged polymers onto supporting mem-branes are reported in (Table 2).

Polysalts, formed from anionic and cationicpolyelectrolytes, would be appropriate forobtaining both highly permeable and highly selec-tive membranes (Semenova et al. 1997) (Table 3).

Ethanol–Water Mixtures: Separation by Pervaporation, Table 2 PV dehydration of 20% aqueous ethanol at 70 �Cusing graft polymer membrane having different charges (Brian et al. 2011)

Host polymer Grafted polymer Graft site charge Separation factor

Polyvinylidene fluoride 4-Vinylpyridine Neutral 9

Polyvinylidene fluoride N-Vinyl-imidazole Neutral 10

Polyvinyl fluoride N-Vinylmethyl-acetamide Neutral 4

Polyvinyl fluoride N-Vinyl-pyrrolidone Neutral 7

Polyacrylonitrile Acrylic acid Neutral 10

Polyacrylonitrile K+ acrylate Anionic 500

Polyvinyl fluoride K+ acrylate Anionic 156

Polyvinylidene fluoride Quaternized 4-vinylpyridine Cationic 175

Polyvinylidene fluoride Quaternized N-vinylimidazole Cationic 61

Polyvinylidene fluoride 4-Vinylpyridine/BrCH2COOH Zwitterionic 76

Polyvinyl fluoride Vinylimidazole/BrCH2COOH Zwitterionic 63

Ethanol–Water Mixtures: Separation by Pervaporation, Table 3 PV dehydration of aqueous ethanol with mem-brane based on various polysalts (Brian et al. 2011)

Polyanion PolycationFeed (wt%water)

Temp.(�C)

Separationfactor

Flux(g/m2h)

Poly(acrylonitrile-co-acrylic acid)

Poly(acrylonitrile-co-vinyl pyridine) 10 – 5000 400

Cellulose-SO3-Na+ Polyethyleneimine 16 50 295 1900

Cellulose-SO3-Na+ PolyDADMAC, linear 16 50 140 3200

Cellulose-SO3-Na+ Same, but branched 16 50 123 4900

Cellulose-SO3-Na+ Poly-N, N-dimethyl-3, 5-

dimethylenepiperidine chloride16 50 123 2700

Aromatic polyamidesulphonate

Polyethyleneimine 20 60 15 300

Poly(acrylic acid) Chitosan

On polysulphone supporting membrane 5 30 1008 132

No supporting membrane 5 30 2216 33

Na+ polystyrenesulphonate

Polyallylamine HC1 6.2 70 70 230

Na+ CMC Chitosan 10 70 1062 1140

Na+ CMC N-Ethyl-4-vinyl-pyridinium bromide 10 70 782 1320

Anionic PVA Cationic PVA

DS 2.3 % DS 2.9 % 4.6 75 2250 378

DS 5.0 % DS 5.2 % 4.6 75 1910 284

Ethanol–Water Mixtures: Separation by Pervaporation 725

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The best performers in terms of flux, which at amaximum of 5 kg/m2h never achieve high values,are charged polymers of one type or another,including polysalts. Anionic and polysalt mem-branes are superior. For anionic polymers, theproton form has a significantly higher flux thanthe metal or quaternary ammonium salt versions,owing to the greater free space within the polymernetwork (Table 4).

Hybrid MembranesMany a times, the polymeric membranes may failto meet the desired separation requirements. Insuch cases, it becomes necessary to add fillermaterials such as ceramics and zeolites to improvethe separation properties of the membrane. Thereare several reports showing good separation per-formance for ethanol/water mixture using zeolitemembranes (Kita et al. 1995; Sano et al. 1994).

Ethanol–Water Mixtures: Separation by Pervaporation, Table 4 Highest fluxes for PV dehydration of aqueousethanol (Brian et al. 2011)

Membrane polymer Mem. type Feed (wt%water) Temp. (�C) Flux (g/m2h)Separationfactor

Nafion-H+ Anionic 4 70 5000 2.5

Cellulose-SO3-Na+ and

polyDADMAC, branchedPolysalt 16 50 4900 125

K+ acrylate graft on poly(vinylfluoride)

Anionic 20 70 4700 156

PEI/PAA on RO membrane Polysalt 10+ 70 4050 1075

Cellulose-SO3-Na+ and

polyDADMAC, linearPolysalt 16 50 3200 140

K+ acrylate graft on PAN Anionic 20 70 3000 500

Ethanol–Water Mixtures: Separation by Pervaporation, Table 5 PV dehydration of ethanol using PVA/inorganichybrid membranes (Brian et al. 2011)

Crosslinker Feed (wt% water) Temp. (�C) Separation factor Flux (g/m2h)

TEOS (160 �C) 15 40 329 50

TEOS (130 �C) 15 40 893 40

PEG blend and TEOS 15 50 300 46

No PEG 15 50 160 500

Poly(acrylic acid) copolymer and TEOS 15 40 250 18

g-Aminopropyl-triethoxysilane 5 50 537 36

Sulphated zirconia 5 50 263 10

10 50 142 105

20 50 86 183

30 50 61 1036

726 Ethanol–Water Mixtures: Separation by Pervaporation

Kita et al. made NaA-type zeolite membrane byhydrothermal synthesis. NaA zeolite membrane isa water-selective membrane, and the PV separa-tion factor of water/ethanol system was over10,000 at 348 K. For ethanol permselective mem-branes, Sano et al. (1994) prepared polycrystallinesilicalite membrane by the hydrothermalsynthesis.

The silicalite membrane showed high ethanolpermselectivity, and a separation factor of 58 wasrealized at 333 K by PV. Silicalite membranesseem to have great potential for the ethanol recov-ery by PV (Table 5).

References

Anne J, Robert C, Pierre LND, Bruno C (2002) Industrialstate-of-the-art of pervaporation and vapour perme-ation in the western countries. JMembr Sci 206:87–117

Binning RC, Lee RJ, Jennings JF, Martin EC (1961) Sepa-ration of liquid mixtures by permeation. Ind Eng Chem53:45

Brian B, Manh H, Zongli X (2011) A review of membraneselection for the dehydration of aqueous ethanol bypervaporation. Chem Eng Process 50:227–235

EP 749351A1 (1996) Device for separating mixtures or forpurifying substances by pervaporation

EP 765682A1 (1997) Apparatus for separating liquidmedia with two membranes having their primary sidesconnected by an intermediate space

EP 811420A1 (1997) Composite membrane for selectiveseparating organic substances by pervaporation

EP 880400A1 (1998) Composite membrane with a supportmembrane made in particular of a microporous material

EP 909209A1 (1999) Pervaporisation and module for car-rying out said process

EP 944575A1 (1999) Esterification of fermentation-derived acids via pervaporation

Graham T (1866) On the absorption and dialytic separationof gases by colloid septa. Philos Mag J Sci 32:401–420

Kataoka T, Tsuru T, Nakao S, Kimura S (1991a) Perme-ation equations developed for prediction of membraneperformance in pervaporation, vapor permeation and

Ethyl Cellulose (EC) Membrane 727

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reverse osmosis based on the solution diffusionmodel. J Chem Eng Jpn 24:326

Kataoka T, Tsuru T, Nakao S, Kimura S (1991b) Mem-brane transport properties of pervaporation inethanolwater system using polyacrylonitrile and cel-lulose acetate membranes. J Chem Eng Jpn 24:334

Kita H, Horii K, Ohtoshi Y, Tanaka K, Okamoto K (1995)Synthesis of a zeolite NaA membranefor pervaporationof water/organic liquidmixtures. JMater Sci Lett 14:206

Kujawski W (2000) Application of pervaporation andvapor permeation in environmental protection. PolJ Environ Stud 91:13–26

Lee CH (1975) Theory of reverse osmosis and some othermembrane permeation operations. J Appl PolymSci 1983

Mikihiro N, Takeo Y, Sin-ichi N (1998) Ethanol/watertransport through silicalite membranes. J Membr Sci144:161–171

Mulder MHV, Smolders CA (1984) On the mechanism ofseparation of ethanol/water mixtures by pervaporationI. Calculations of concentration profiles. J Membr Sci17:289

Paul DR, Paciotti JD (1975) Driving force for hydraulicand pervaporative transport in homogeneous mem-branes. J Polym Sci 13:1201

Sano T, Yanagishita H, Kiyozumi Y, Mizukami F, Haraya K(1994) Separation of ethanol/water mixture by silicalitemembrane on pervaporation. J Membr Sci 95:221

Semenova SI, Ohya H, Soontarapa K (1997) Hydrophilicmembranes for pervaporation: an analytical review.Desalination 110:251–286

Ethyl Cellulose (EC) Membrane

Hongyang Ma, Benjamin S. Hsiao andBenjamin ChuDepartment of Chemistry, Stony BrookUniversity, Stony Brook, NY, USA

Ethyl cellulose (EC) membrane has mainly beenused for separation of light gases, such as He, O2,N2, CH4, CO2, propane, and propylene. Thehydroxyl groups on the glucose unit of cellulose

O

OH

OHHOO

O

HO OHO

OH n

Etherifica

Ethyl Cellulose (EC) Membrane, Fig. 1 Etherification of c

can be substituted with epoxy groups up to about55 %, where ethyl cellulose exhibits good thinfilm-forming ability, excellent mechanical proper-ties and durability, outstanding gas separationperformance, and cost-effectiveness. Ethyl cellu-lose can be dissolved in organic solvents (e.g.,chloroform) and ethyl cellulose thin film can beformed via solvent evaporation. The “nonporous”ethyl cellulose membrane has been used to sepa-rate light gases, where the solubility coefficientand the diffusion coefficient are two major param-eters to determine the membrane performance forgas separation. This performance is alsoinfluenced by the content of epoxy groups,which vary with different manufacturers (Houdeet al. 1997; Li et al. 2001; Ito et al. 1989) (Fig. 1).

Another important application of ethyl cellu-lose is in film coating of drug tablets. Although thetablet coating does not have a therapeutic value, itdoes offer the psychological importance in aidingpatients to take the medication. The incorporationof color, flavor, and other additives in the ethylcellulose-coated tablets can improve esthetic andorganoleptic values and improve stability andhandling capability and drug release properties.For example, ethyl cellulose is a water-insolublepolymer, which can be used to protect the hydro-lysis of the drug by surface coating. The drugrelease rate can be adjusted by varying the thick-ness of the film and/or the composition of hydro-phobic and hydrophilic components in the coating(Sakellariou et al. 1995). Similarly, ethyl cellulosecan be used to formulate microcapsules where thetarget drug(s) can be tailored with controlledrelease rate, as well as improved efficacy, safety,processibility, and stability (Rogers et al. 2012). Itis interesting to note that hydrophobic ethyl cel-lulose and hydroxypropyl cellulose, which is

O

OR

ORROO

O

RO ORO

OR n

tion

R = H or CH2CH3

ellulose with maximum content of epoxy of about 55 %

728 Ethylene Off-Gas

more hydrophilic, are often combined togethersynergistically for varying biomedical applica-tions (Donbrow et al. 1980).

References

Donbrow M, Sanuelov D (1980) Zero order drug deliveryfrom double-layered porous films: release rate profilesfrom ethyl cellulose, hydroxypropyl cellulose andpolyethylene glycol mixtures. J Pharm Pharmacol32:463–470

Houde AY, Stern SA (1997) Solubility and diffusivity oflight gases in ethyl cellulose at elevated pressures:effects of epoxy content. J Membr Sci 127:173–183

Li XG, Kresse I, Xu ZK, Springer J (2001) Effect oftemperature and pressure on gas transport in ethyl cel-lulose membrane. Polymer 42:6801–6810

Ito A, Hwang ST (1989) Permeation of propane and pro-pylene through cellulosic polymer membranes. J ApplPolym Sci 38:483–490

Sakellariou P, Rowe RC (1995) Interactions in cellulosederivative films for oral drug delivery. Prog Polym Sci20:889–942

Rogers TL, Wallick D (2012) Reviewing the use ofethylcellulose, methylcellulose and hypromellose inmicroencapsulation. Part 3: applications for microcap-sules. Drug Dev Ind Pharm 38:521–539

Ethylene Off-Gas

Paola BernardoInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Ethylene off-gas is a light gas mixture containingvarious concentrations of ethylene, propylene,hydrogen, methane, ethane, propane, and minoramounts of higher hydrocarbons, nitrogen, andother trace components.

The main ethylene-bearing off-gas streams areproduced during the petroleum refining from fluidcatalytic cracking (FCC) units and delayedcokers. FCC units alone produce significantamounts of ethylene and hydrogen, owing to thehigh feed flow rates processed. Other sources forethylene off-gas streams are the deep catalyticcracking processes.

Refinery and petrochemical ethylene-containing off-gas streams usually contain vary-ing amounts of heavy C4–C10 hydrocarbons,including alkanes, olefins, and aromatics, togetherwith small quantities of water, nitrogen oxides,carbon monoxide, carbon dioxide, acetylene,methylacetylene, propadiene, butenes, and higherhydrocarbons. FCC off-gases are available at10–17 bar and typically contain 8–18 vol.% ofethylene, 3–9 vol.% of propylene, and 12–20vol.% of hydrogen, depending on the FCC feedcomposition and cracking severity (Netzer 2003).In general, ethylene-bearing off-gas streams con-tain relatively dilute concentrations of olefins,such as ethylene, with methane, ethane, andhydrogen which are potentially recoverable.

Hydrogen and light hydrocarbons (ethylene,propylene, and LPG) have an increasing world-wide demand. Being more valuable as chemicaland plastic feedstocks than they are as fuels, therecovery from off-gas streams of these valuablecompounds represents an interesting possibility toincrease revenue in refineries (Hoffmann andKaufmann 2012). Indeed, refinery and petro-chemical ethylene off-gas streams are typicallyburnt in flare stacks or as fuel, since the recoveryof the olefins by conventional means, such asfractionation, is considered as not economicallyviable. Considering the high cost of producingethylene by thermal cracking of hydrocarbonfeedstocks, which is the primary production pro-cess for ethylene, the recovery of olefins repre-sents a substantial conservation of resources(Eldridge 1993). The available separationmethods are absorption, adsorption, distillation,and membrane separation. Membrane technologyrepresents a cost-effective, flexible, and safe pro-cess for recovering ethylene and associatedhydrogen from refinery and petrochemicaloff-gas streams (Ohlrogge et al. 2010).

The use of a membrane system for vapor–gasseparation allows valuable feedstocks to be recov-ered and recycled to the polymerization section inpolyethylene and polypropylene production(Baker et al. 1998). Polyolefin plants involve pre-sent losses of monomers and other hydrocarbonfeedstocks (typically $1–$3 million per year inone plant). Vent streams, containing monomer

Ethylene Production by Membrane Operations 729

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contaminated with light gases (e.g., N2 and H2),are usually flared. Monomer recovery withhydrocarbon-selective rubbery membranes is thelargest application of vapor separation mem-branes. The vent stream is compressed and cooledto condense hydrocarbons. The gas not con-densed, still containing a significant amount ofhydrocarbons, is sent to the membrane systemand separated into a hydrocarbon-enriched streamas permeate and a nitrogen stream. The permeateis recycled to the compressor and then to thecondenser where the hydrocarbon is recovered;the nitrogen stream is recycled to the degassingbin (Baker et al. 1998).

Analogous membrane processes are applied torecover ethylene in ethylene oxide and in vinylacetate production. The purge gas for a typicalethylene oxide plant contains approximately20–30 % ethylene, 10–12 vol.% argon, 1–10vol.% carbon dioxide, 1–3 vol.% ethane,50 vol.% methane, and 4–5 vol.% oxygen.A similar vent gas mixture is produced in thevinyl acetate process. A membrane recovery unitfed with these purge streams will produce anethylene-enriched permeate stream and anargon-enriched residue stream (Jacobs and Billig2005). These systems achieve an ethylenerecovery greater than 70 %, partly recover meth-ane (the diluent gas), and reduce hydrocarbonburning.

References

Baker RW, Wijmans JG, Kaschemekat JH (1998) Thedesign of membrane vapour-gas separation systems.J Membr Sci 151:55–62

Eldridge RB (1993) Olefin/paraffin separation technology:a review. Ind Eng Chem Res 32:2208–2212

Hoffmann K, Kaufmann D (2012) Recovery of valuableolefin products from refinery off-gas streams. ChemNews 14–19

Jacobs ML, Billig BJ (2005) Achieving ethylene effi-ciency. Hydrocarb Eng

Netzer D (2003) A combination process for manufacturingethylene, ethylbenzene and styrene. EP Patent1,017,651 B1

Ohlrogge K, Wind J, Brinkmann T (2010) Membranes forrecovery of volatile organic compounds. In: Drioli E,Giorno L (eds) Comprehensive membrane science andengineering, vol 2. Elsevier, Oxford, pp 213–242

Ethylene Production by MembraneOperations


Ethylene (C2H4), also called ethene, is a colorlessand flammable gas derived from natural gas andpetroleum. Being particularly reactive, ethylene rep-resents one of the largest volume compounds and akey building block in the petrochemical industry,with a worldwide demand that has grown steadily.

The conventional ethylene production processis steam cracking. A hydrocarbon feedstock (suchas naphtha, liquefied petroleum gas, ethane, pro-pane, or butane) is thermally cracked in the pres-ence of steam, producing a mixture of hydrogen,methane, ethylene, ethane, and heavier compo-nents. Most of the dilution steam and heavierproducts are condensed by a rapid quenching,while the cooled gases are compressed, sent to ascrubbing for the acid gas removal, and subcooledfor the successive separations. A distillation trainperforms the separation and purification of thegaseous products, representing an energy con-suming step, highly heat integrated and requiringrefrigeration (Ren et al. 2006). This provides astrong drive for evaluating alternative methodsfor separation and recovery purposes. Coke for-mation requires discontinuous operation for reac-tor cleanup. Different wastes are generatedincluding caustic scrubber effluent, dilutionsteam condensate, coke and tar at the condensateseparators, and vent gases generated during start-ups and shutdowns. Overall, this process is verycapital intensive and energy consuming (Renet al. 2006). This type of hydrocarbon feedstockaffects the process capital cost as well as theamounts and types of waste streams. Cleaner andsimplified processes are related to the use of eth-ane. Multiple companies are considering thisoption, driven by the large volumes of shale gasbeing recently recovered which provides sizablevolumes of very low-cost ethane (Armor 2013).

730 Ethylene/Ethane Separation by Membranes

The integration of membrane separation systemsand of membrane contactors (such as membranedistillation, membrane strippers and scrubbers,etc.) andmembrane reactors might beneficially con-tribute to redesign industrial operations. Indeed,membrane technology has many advantages overother conventional technologies, including lowercapital and operating costs, low maintenance, lowspace requirements, flexibility, and ease of installa-tion and operation (Strathmann 2011).

An ethylene production cycle by steam crack-ing was redesigned by integrating different mem-brane systems (▶ gas separation modules,▶microfiltration, and membrane contactors) intoa reference plant producing 800,000 t/year of eth-ylene and consuming ca. 30 GJ/t ethylene(Bernardo et al. 2004). Membrane gas separationto remove hydrogen from compressed gas allowsa debottlenecking, reducing significantly theenergy required in the cryogenic distillation forhydrogen/methane separation, permitting lesssevere operating conditions with respect to theconventional cycle (Engler and Dupuis 2000).Gas separation was also proposed to produceoxygen-enriched air to be used instead of air forcombustion and/or decoking. Hydrocarbonremoval from water streams using membranecontactors, capable to operate more efficientlywith respect to conventional scrubbing units(Gaeta 2009), could achieve a removal greaterthan 90 %. Membrane contactors could be alsoapplied for acid gas removal from the furnaceeffluent. Water used for scrubbing of the finestfraction of coke particles during decoking can beprocessed in a microfiltration membrane unit,recovering up to 90 % of the water and allowingits reuse in the plant. The replacement of tradi-tional operations with membrane units, such asmembrane contactors for washing water purifica-tion in the “hot section” and the membrane gasseparation for the hydrogen recovery, determine asmaller exergetic loss with respect to the conven-tional industrial cycle (Bernardo et al. 2004).

Membrane reactors with inorganic membranesable to operate at high temperature were investi-gated for alternative ethylene productions,benefitting from the product continuous removalto shift a chemical equilibrium or from a reactant

continuous feeding to control the reaction selec-tivity. Some examples, related to the conversionof ethane, include the dehydrogenation by meansof hydrogen permeable metallic membranes(Galuszka et al. 2008) and the exothermic oxida-tive dehydrogenation using perovskite mem-branes, which operate under a mixedion-electron conduction (Lobera et al. 2011).

References

Armor JN (2013) Emerging importance of shale gas to boththe energy & chemicals landscape. J Energy Chem22:21–26

Bernardo P, Criscuoli A, Clarizia G, Barbieri G, Drioli E,Fleres G, Picciotti M (2004) Applications of membraneunit operations in ethylene process. Clean TechnolEnviron Policy 6(2):78–95

Engler Y, Dupuis G (2000) Process for recovering olefins.U.S. Patent 6,141,988

Gaeta S (2009) Membrane contactors in industrial applica-tions. In: Drioli E, Giorno L (eds) Membrane opera-tions: innovative separations and transformations.Wiley-VCH, Weinheim, pp 499–512

Galuszka J, Giddings T, Clelland I (2008) Catalytic dehydro-genation of ethane in hydrogen membrane reactor. In:Bose AC (ed) Inorganic membranes for energy and envi-ronmental applications. Springer, NewYork, pp 299–311

Lobera MP, Escolástico S, Serra JM (2011) High ethyleneproduction through oxidative dehydrogenation of eth-ane membrane reactors based on fast oxygen-ion con-ductors. Chem Cat Chem 3:1503–1508

Ren T, Patel MK, Blok K (2006) Olefins form conventionaland heavy feedstocks: energy use in steam cracking andalternative processes. Energy 31(4):425–451

Strathmann H (2011) Introduction to membrane scienceand technology. Wiley-VCH, Weinheim

Ethylene/Ethane Separationby Membranes


The separation of ethylene from ethane and pro-pylene from propane is a challenging operation. Itis involved in the production of light olefins, such

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Ethylene/Ethane Separation by Membranes 731

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as ethylene and propylene, which represent themost important building blocks in the petrochem-ical industry. Cryogenic distillation, convention-ally applied to this separation, is highly energyintensive, requiring expensive tall columns, oper-ated under low temperature and high pressure,with very high reflux ratios and requiring ethyl-ene/propylene refrigeration systems forlow-temperature cooling (Eldridge 1993).

Due to the complexity and difficulty of theseparation process, mixtures including olefinsproduced in the petroleum and refining processare often used as fuel. Membrane separation, rec-ognized as a low-energy method, appears the mostpromising among alternative separation tech-niques, such as extractive distillation, adsorption,and absorption, to recover these valuable com-pounds (Rungta et al. 2013). Polymeric mem-branes, cheap and with an easy processability,have low olefin/paraffin separation factor, espe-cially those based on rubbery polymers (Rungtaet al. 2013). The experimentally observed ethyl-ene/ethane and propylene/propane upper bound,based on the most reliable data of various poly-meric membranes, were presented in two criticalreviews (Rungta et al. 2013; Burns and Koros2003, respectively). Glassy polymers have beenstudied intensively for this separation. However,when applied to mixtures and/or at high gas activ-ities, these materials are prone to plasticizationwhich causes swelling of the polymer matrix andresults in a higher permeability coupled with aloss of selectivity. Strategies to overcome plasti-cization include thermal curing and chemicalcross-linking, which reduce the polymer free vol-ume, and the addition of nanofillers (Gohet al. 2011; Ploegmakers et al. 2013).

Facilitated transport membranes based on thep-complexation mechanism are broadly investi-gated for olefin/paraffin separation (Faiz and Li2012). The specific interaction between the hybridmolecular orbitals of the olefins and the atomicorbitals of the transition metals results in revers-ible chemical bonds stronger than those formed byvan der Waals forces, but can be broken by simplyincreasing the temperature or decreasing the pres-sure. These systems allow high selectivity forthe bound component. Silver-based facilitated

transport membranes were studied more exten-sively due to low-cost and relatively weak inter-actions with olefins (possible decomplexation).However, carrier poisoning and short life span ofthe polymeric membranes are typically reported(Rungta et al. 2013). Chitosan-based membranes,impregnated with silver nitrate as a facilitationagent, showed performance stability over thou-sands of hours of operation in field tests (Kamzaet al. 2013). Ionic liquids, selected as additives forfacilitated transport membranes, have a negligiblevapor pressure that avoids solvent losses by evap-oration and offer more affinity for the olefinscompared to the paraffins, providing stability tothe metallic cation dissolved inside and acting as amedium for facilitated transport with mobile car-rier (Fallanza et al. 2013).

Carbon molecular sieve membranes showedthe potential to surpass the polymeric ethylene/ethane upper bound performance (Rungtaet al. 2013). These membranes are usually pre-pared by the pyrolysis of polymeric precursorssuch as polyimide materials. However, majorissues are represented by their brittleness andpoor mechanical strength.

In the field of inorganic membranes, metalorganic frameworks were recently considered forpreparing membranes to be applied to the olefin/paraffin separation (Bux et al. 2011).

The concept of hybrid processes, combining amembrane system with a conventional one, hasdrawn significant attention for the olefin/paraffinseparation. Savings in total costs and energy(up to 30 %) could be obtained with a mem-brane/distillation hybrid system (Caballeroet al. 2009).

References

Burns RL, Koros WJ (2003) Defining the challenges forC3H6/C3H8 separation using polymeric membranes.J Membr Sci 211:299–309

Bux H, Chmelik C, Krishna R, Caro J (2011) Ethene/ethane separation by the MOF membrane ZIF-8:molecular correlation of permeation, adsorption, diffu-sion. J Membr Sci 369:284–289

Caballero JA, Grossmann IE, Keyvani M, Lenz ES(2009) Design of hybrid distillation – vapor membrane

Feed liquid

Membrane

vacuum

Evapomeation (EV),Fig. 1 Principle ofpervaporation (PV)

Membrane

Feed liquid

vacuumEvapomeation (EV),Fig. 2 Principle ofevapomeation (EV)

732 Evapomeation (EV)

separation systems. Ind Eng Chem Res48(20):9151–9162

Eldridge RB (1993) Olefin/paraffin separation technology:a review. Ind Eng Chem Res 32:2208

Faiz R, Li K (2012) Polymeric membranes for light olefin/paraffin separation. Desalination 287:82–97

FallanzaM, Ortiz A, Gorri D, Ortiz I (2013) Polymer–ionicliquid composite membranes for propane/propyleneseparation by facilitated transport. J Membr Sci444:164–172

Goh PS, Ismail AF, Sanip SM, Ng BC, Aziz M (2011)Recent advances of inorganic fillers in mixed matrixmembrane for gas separation. Sep Purif Tech81:243–264

Kamza A, Keyvani M, Towe G (2013) Stable facilitatedtransport membrane for olefin/paraffin separation. In:Fuels and petrochemicals division, AIChE SpringMeeting and Global Congress on Process Safety, SanAntonio. 28 Apr – 2 May 2013

Ploegmakers J, Japip S, Nijmeijer K (2013) Mixed matrixmembranes containing MOFs for ethylene/ethane sep-aration. Part A: membrane preparation and characteri-zation. J Membr Sci 428:445–453

Rungta M, Zhang C, Koros WJ, Xu L (2013) Membrane-based ethylene/ethane separation: the upper bound andbeyond. AIChE J 59(9):3475–3489

Evapomeation (EV)

Tadashi UragamiOrganization for Research and Development ofInnovative Science and Technology (ORDIST),Kansai University, Suita, Osaka, Japan

Presently, pervaporation (PV) is applied as thechosen membrane separation technique for theseparation of water/organic, organic/water, andorganic/organic mixtures. However, it seemsthat conventional PV is not the most efficientmembrane separation process for the treatmentof some liquid mixtures as follows. Because thepolymer membranes used in PV are directly incontact with the liquid feed solutions as shown inFig. 1, however, specifically designed chemicaland physical properties of the membrane are oftenimpaired by swelling or shrinking of the mem-brane due to sorption of the feed components.Swelling or shrinking of the polymer membranesis disadvantageous for the membrane perfor-mance with respect to the separation of mixtures.

A novel membrane separation technique knownas “evapomeation (EV)” (Uragami et al. 1988;Uragami and Saito 1989; Uragami 1991, 1992,1993a, b; Uragam 1994; Uragami 1998, 2005,2006a, b, 2008) makes use of the advantages ofPV but reduces the negative effects of swelling onmembrane performance. In this technique, thefeed solution is fed to the membrane withoutdirectly contacting the polymer membrane. Thisis accomplished by vaporizing the liquid feed sothat only vapor is supplied to the polymer mem-brane as shown in Fig. 2. Therefore, swelling orshrinking of the polymer membranes due to con-tact with the feed solutions is minimized.

The advantages of EV compared to PV are asfollows:

1. In the EV process, membranes are not in directcontact with liquid feed mixtures as onlyvapors are supplied to the membranes. Accord-ingly, any swelling or shrinking of the mem-brane due to the feed mixtures is minimized,and consequently an improvement in mem-brane performance may be expected.

Evaporation Casting, Fig. 1 Evaporation castingprocess

Evaporation Casting 733

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2. Because the organic liquid mixtures are vapor-ized, interactions between component mole-cules are significantly weakened, andconsequently the separation performance isremarkably improved.

3. In EV, contaminants in a liquid feed mixture,such as macromolecular solutes, can lead tofouling of the membrane; this problem isavoided in EV.

4. During EV, both temperature of the feed solu-tion and the membrane surroundings can becontrolled; hence, an improvement in the per-meation and separation characteristics of themembrane can be achieved.

References

Uragam T (1994) Separation method of mixed solutions.Japanese Patent 1,883,353

Uragami T (1991) Method of separating a particular com-ponent from its liquid solution. US Patent 4,983,303,European Patent 0,273,267

Uragami T (1992) Separation of organic liquid mixturesthrough chitosan and chitosan derivative membranesby pervaporation and evapomeation methods. In: BrineCJ, Sandford PA, Zikakis JP (eds) Advance in chitinand chitosan. Elsevier Applied Science, Oxford,pp 594–603

Uragami T (1993a) Method of separating a particular com-ponent from its liquid solution. European Patent0,273,267

Uragami T (1993b) Method of separating a particular com-ponent from its liquid solution. Brazilian PatentP-8707041-3

Uragami T (1998) Structures and properties of membranesfrom polysaccharide derivatives. In: Dumitriu S(ed) Polysaccharide – structural diversity and func-tional versatility. Marcer Dekker, New York/Basel/Hong Kong, pp 887–924

Uragami T (2005) Structures and functionalites of mem-branes from polysaccharides derivatives. In: DumitriuS (ed) Polysaccharides structural diversity and func-tional versatility, 2nd edn. Marcer Dekker, New York/Basel/Hong Kong, pp 1087–1122

Uragami T (2006a) Polymer membranes for separation oforganic liquid mixtures. In: Yampolskii Y, Pinnau I,Freeman BD (eds) Materials science of membranesfor gas and vapor separation. John Wiley & Sons,Chichester, pp 355–372

Uragami T (2006b) Separation materials derived fromchitin and chitosan. In: Uragami T, Tokura S (eds) Mate-rial science of chitin and chitosan, KODANSHA.Springer, Tokyo/Berlin/Heidelberg/New York,pp 113–163

Uragami T (2008) Structural design of polymer membranesfor concentration of bio-ethanol. Polym J 40:485

Uragami T, Saito M (1989) Analysis of permeation andseparation characteristics: a new technique for separa-tion of aqueous alcoholic solution through alginic acidmembranes. Sep Sci Technol 24:54

Uragami T, Saito M, Takigawa K (1988) Comparison ofpermeation and separation characteristics for aqueousalcoholic solutions by pervaporation and newevapomeation methods through chitosan membranes.Makromol Chem Rapid Commun 9:361

Evaporation Casting

Francesco GalianoInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

Evaporation casting or dry-casting involves theevaporation of a solvent (or a mix of solvents)from a starting solution and the subsequent for-mation of a polymeric membrane by precipitation.In this process a polymer is dissolved in a suitablesolvent and the solution obtained is spread outacross an appropriate support. Then, the solventis left to evaporate, in inert atmosphere or con-trolled environment, inducing the polymer precip-itation and generating a membrane generally witha dense structure (Fig. 1). In evaporation casting,in fact, the precipitation process is much slowerthan the precipitation obtained by immersion cast-ing. As a consequence, the membranes present,usually, an isotropic and less porous structure.Some other components, called commonlynonsolvents, can be also added to the initial poly-meric solution. In this case, the solvent, the more

734 Ex Situ Zeolite Synthesis

volatile element of the system, will evaporatefaster leading to a higher polymer/nonsolventconcentration responsible for the polymer precip-itation. The evaporation process can proceed untilthe membrane has completely formed or it can bestopped by immersing the cast film in a coagula-tion bath containing a nonsolvent (Baker 2004).By prolonging or reducing the evaporation timebefore immersion in the coagulation bath, it ispossible to tune the pore size of the membrane.When the starting polymeric solution is cast ondifferent support, as porous thin films or otherkinds of membranes, an asymmetric membranewith a dense skin layer deposited on a poroussupport can be obtained.

References

Baker W (2004) Membrane technology and applications,2nd ed. Wiley, West Sussex, England, Chapter 3,pp 112–113

Ex Situ Zeolite Synthesis

▶ Seeded Hydrothermal Synthesis for ZeolitePreparation

External Coagulant in FiberPreparation

Khayet MohamedDepartment of Applied Physics, UniversidadComplutense de Madrid, Madrid, Spain

External coagulant is a non-solvent liquid or amixture of solvent/non-solvent in a collectingbath (coagulation bath) used for fabrication offibers by the dry-/wet-spinning technique or thewet-spinning technique. Coagulation of the exter-nal surface of the nascent fiber can start immedi-ately after its extrusion from the spinneret (wetspinning) or after the as-spun fiber crosses an air

gap (i.e., distance between the spinneret and thecoagulation bath) through which the external sur-face experiences coalescence and orientation ofpolymer aggregates (wet/dry spinning). In gen-eral, the nascent fiber is partially coagulated bythe internal coagulant (bore liquid) and/or throughthe air gap by solvent evaporation, and finally thecoagulation is completed in the coagulation bathby an external coagulant.

The type of the external coagulant affects con-siderably the structural morphology of theresulted fiber. Usually, water is the preferredexternal coagulant because of its low cost andenvironmental friendliness. The proper choice ofthe coagulant is very important because the rate ofdemixing (i.e., phase separation) and the resultantinner and outer surface structures of the hollowfiber strongly depend on the chemistry and thecomposition of the coagulant. The molecularsizes and solubility parameters of the used solventand the internal and the external coagulant playimportant roles on hollow fiber morphology. Sol-vent of a large size may have difficulties to leachout during the coagulation step. The difference insolubility parameters between the spinning dopesolution and the bore liquid (internal coagulant) orthe external coagulant affects the coagulation rateand subsequently the porosity of the spun hollowfiber membrane.

The interactions between solvent and externalcoagulant and between solvent and polymer playimportant roles affecting the rate of polymer pre-cipitation. The temperature of the external coagu-lant affects also the polymer precipitation rate.The maximum amount of solvent that can beadded to the non-solvent in the coagulation bathcan be roughly determined by the position of thebinodal (liquid-liquid boundary separatingthe miscible region from the immiscible one) inthe ternary phase diagram presenting the compo-sition of the polymer, solvent, and non-solvent.When the binodal shifts toward the polymer/sol-vent axis, more solvent can be added in the exter-nal coagulant. It is also possible to change fromporous to nonporous external fiber surface byadding solvent to the coagulation bath (Mulder1992). For example, when a strong coagulantsuch as water for poly(vinylidene fluoride),

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External Coagulant in Fiber Preparation,Fig. 1 Cross-sectional scanning electron microscopy(SEM) structure of PVDF hollow fiber membranes pre-pared with different coagulants: (a) water coagulant;

(b) internal coagulant water and external coagulant 50 %ethanol in water (Reprinted from (Khayet et al. 2002).Copyright 2002, with kind permission from Elsevier)

External Coagulant in Fiber Preparation 735

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PVDF, polymer is used for hollow fiber fabrica-tion, a dense and smooth surface with no obviouspores is formed. However, when a weaker coagu-lant is employed such as methanol, which may bemixed with water, the roughness of the formedhollow fiber surface may increase considerablydue partially to the formation of pores. These arethe reasons for using the coagulants: 80 wt.%N-methylpyrrolidone (NMP) aqueous solutionby Wang et al. (2008) for preparation of PVDFhollow fiber membranes and 80 wt.% methanolaqueous solution by Bonyadi and Chung (2007)for fabrication of dual hydrophilic/hydrophobichollow fiber membranes for direct contact mem-brane distillation (DCMD) process. Figure 1shows the effects of the external coagulant onthe cross-sectional structure of porous hydropho-bic hollow fiber membranes (Khayet et al. 2002).Water and 50 % (by volume) ethanol in waterwere used as internal and external coagulants. InFig. 1a prepared with water as internal and exter-nal coagulants, long finger-like voids are formednear the inner surface, while smaller cavities areformed near the outer surface. Between the innerand outer layers, sponge-like structure appears. InFig. 1b the outer layer is eliminated when ethanolis added to the external coagulant. The addition ofethanol delays the external coagulation processand the finger-like structure changes to a sponge-like structure. The slow coagulation of the hollow

fiber membranes can be explained on the basis ofthe mutual diffusivity of solvent/non-solventexchange and solubility parameters of the mate-rials involved in hollow fiber spinning. The addi-tion of ethanol in water reduces the diffusion ofnon-solvent into the nascent hollow fiber mem-brane and consequently decreases the rate of pre-cipitation. Khayet et al. (2002) observed anincrease of the external surface roughness ofPVDF hollow fibers when ethanol was added tothe external coagulant and attributed the result tothe increase of the pore size at the outer surface asethanol was added in the coagulation bath.

References

Bonyadi S, Chung TS (2007) Flux enhancement in mem-brane distillation by fabrication of dual layerhydrophilic-hydrophobic hollow fiber membranes.J Membr Sci 306:134–146

Khayet M, Feng CY, Khulbe KC, Matsuura T (2002) Prep-aration and characterization of polyvinylidene fluoridehollow fiber membranes for ultrafiltration. Polymer43:3879–3890

Mulder M (1992) Basic principles of membrane technol-ogy. Kluwer Academic Publishers, Dordrecht

Wang KY, Chung TS, Gryta M (2008) Hydrophobic PVDFhollow fiber membranes with narrow pore size distri-bution and ultra-skin for the fresh water productionthrough membrane distillation. Chem Eng Sci63:2587–2594

736 Extracellular Polymeric Substance (EPS)

Extracellular Polymeric Substance(EPS)

Loreen O. VillacorteFMC Technologies, Separation Innovation andResearch Center, Arnhem, The Netherlands

Extracellular polymeric substances or EPSs arebiosynthetic polymers from prokaryotic(bacteria, archaea) and eukaryotic (algae, fungi)microorganisms, which either form (loose ortight) slimes around the microbial cells or areexcreted as discrete gels to the surrounding envi-ronment. Typically, EPSs are heterogeneous mix-tures of polysaccharides, proteins, nucleic acids,lipids, and other polymeric compounds. Thehighly diverse chemical composition of EPS is aresult of the different processes related to theirproduction and their immediate environment:active microbial secretion, shedding of cell sur-face materials, cell lysis, and adsorption from theenvironment (Wingender et al. 1999).

EPSs are often associated with the formation ofbiofilms and microbial aggregates. In biofilm sys-tems, they are mainly responsible for binding cellsand other particulate materials together and to thesolid-liquid interface (Characklis and Wilderer1989). In surface water sources, suspended EPSsare responsible for the formation of large aggre-gates of organic and inorganic suspended mate-rials including living and dead microorganisms inthe water (e.g., marine snow or mucilage). Theseaggregates are held together by a type of EPSmainly comprising of surface-active, algae-derived anionic polysaccharides collectivelyknown as transparent exopolymer particles(Alldredge 2002). The latter has been recentlyidentified as one of the major initiators of aquaticbiofilms as it can form conditioning films on thesolid-liquid interface conducive for bacterialattachment (Bar-Zeev et al. 2012).

Over the years, the important role of EPS in themicrobial activities in natural aquatic systems iswidely recognized. Moreover, it has also beenidentified to cause serious problems in technicalsystems ranging from the shipping industry,

power industry, microelectronics, and food indus-tries to water purification (Flemming et al. 2009).In water supply systems, such problems are due toeither organic or biological fouling of reservoirs,pipelines, media filters, and separationmembranes.

EPS accumulation in membrane filtration sys-tems can cause increase of operating pressure andcleaning frequency due to blockage of membranepores as well congestion along the feed channel(Flemming et al. 1997). In NF/RO systems, it candirectly cause decline in permeate water quality(e.g., salt rejection) due to hindered back diffusionof rejected salts and can indirectly cause mem-brane damage due to long exposure of cleaningchemicals and very high feed channel pressuredrop (Vrouwenvelder et al. 2011). It has beenproposed that the removal of planktonic EPSfrom the feedwater by pretreatment with dissolvedair flotation (DAF) and/or UF/MFmembranes caneffectively minimize biofouling in NF/RO sys-tems. However, this is still subject to extensivedebate and investigations.

During algal blooms, very sticky EPS materials(including TEPs) produced by phytoplankton andbacterioplankton can cause physically irreversible(or non-backwashable) fouling in dead-end MF/UFsystems (Villacorte et al. 2010). In this case, chem-ically enhanced backwashing (CEB) can be effec-tive in restoring membrane permeability. However,applying an optimal dose of coagulant(conventional or in-line) prior to membrane filtra-tion has been found to effectivelyminimize the needfor chemical cleaning (e.g., Schurer et al. 2012).

Cross-References

▶Transparent Exopolymer Particle

References

Alldredge AL (2002) Marine snow. In: Nybakken JW,Broenkow WW, Vallier TL (eds) Interdisciplinaryencyclopedia of marine sciences. Grolier AcademicReference, Danbury

Bar-Zeev E, Berman-Frank I, Girshevitz O, BermanT (2012) Revised paradigm of aquatic biofilm forma-tion facilitated by microgel transparent exopolymerparticles. Proc Natl Acad Sci USA 109(23):9119–9124

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Characklis WG, Wilderer PA (1989) Glossary. In:Characklis WG, Wilderer PA (eds) Structure and func-tion of biofilms. Wiley, Chichester, pp 369–371

Flemming H-C, Schaule G, Griebe T, Schmitt J,Tamachkiarowa A (1997) Biofouling – the Achillesheel of membrane processes. Desalination 113:215–225

Flemming H-C, Murthy PS, Venkatesan R, Cooksey KE(eds) (2009) Marine and industrial biofouling,vol 4, Springer Series on Biofilms. Springer, Berlin

Lens P, O’Flaherty, Moran AP, Stoodley P, Mahony T (eds)(2003) Biofilms in medicine, industry and environmen-tal biotechnology: characteristics, analysis and control.IWA Publishing, London

Schurer R, Janssen A, Villacorte L, Kennedy MD(2012) Performance of ultrafiltration & coagulation inan UF-RO seawater desalination demonstration plant.Desalination Water Treat 42:57–64

Villacorte LO, Schurer R, Kennedy MD, Amy G,Schippers JC (2010) The fate of transparentexopolymer particles in integrated membrane systems:a pilot plant study in Zeeland, The Netherlands. Desa-lination Water Treat 13:109–119

Vrouwenvelder JS, Kruithof J, Van Loosdrecht M (2011)Biofouling of spiral wound membrane systems. IWAPublishing, London

Wingender J, Neu TR, Flemming H-C (eds) (1999) Micro-bial extracellular polymeric substances: characteriza-tion, structure, and function. Springer, Berlin

Extracorporeal BALs

▶Bioartificial Liver Support System (BLSS)

Extracorporeal Blood OxygenationDevices, Membranes for

Mónica Faria1 and Maria Norberta De Pinho21Chemical Engineering, Columbia University,New York, NY, USA2Chemical Engineering, Instituto SuperiorTécnico, Universidade de Lisboa, Lisbon,Portugal

Introduction to Extracorporeal BloodOxygenation

In physiology, respiration is defined as the trans-port of oxygen from the outside air to the cells

within tissues and the transport of carbon dioxidein the opposite direction. The heart and lungswork together to ensure the circulation and theexchange of the respiratory gases. During circu-lation, the heart pumps oxygen-depleted blood tothe lungs and then receives oxygen-enrichedblood from the lungs for distribution to the restof the body. In the lungs, the gas exchange pro-cess takes place in grapelike clusters of air sacsknown as the alveoli. There are about 300 millionalveoli in the lungs with a combined surface areaof approximately 70 m2. This is the area throughwhich the blood–gas exchange takes place(Weibel 2009). Fresh air entering the lung carriesoxygen with a partial pressure of oxygen (PO2) of160 mmHg; the presence of moisture in the lungalveoli results in reduction of the PO2 to104 mmHg. The same air that enters the lungcarries carbon dioxide with a partial pressure ofcarbon dioxide (PCO2) of 0.3 mmHg. The carbondioxide delivered to the lung from the blood inthe venous ends of the pulmonary capillariesraises the PCO2 in the alveoli to 40 mmHg(McArdle et al. 2005). At the arterial ends ofthe pulmonary capillaries, oxygen diffuses fromthe air in the alveoli into the blood, and carbondioxide diffuses from the blood into the alveolibecause of differences in partial pressures. Theheart then receives the oxygenated blood fromthe lungs and distributes it to tissues in the rest ofthe body. In most tissues, the PO2 is less than40 mmHg and the PCO2 is greater than45 mmHg so, because of the differences in partialpressures, oxygen diffuses out of the arterial endsof tissue capillaries into the cells, and carbondioxide diffuses out of the cells into the blood.The heart pumps the oxygen-depleted blood backto the venous ends of the pulmonary capillariesentering the alveoli with a PO2 of 40 mmHg andPCO2 of 45 mmHg (Fox 2007).

In June of 2011, the World Health Organiza-tion classified heart diseases as the worldwidenumber one cause of death (WHO 2011). Thetreatment of many cardiac-related diseases suchas coronary artery bypass grafting, valve repair,and aortic aneurysm repairs involves complexopen-heart procedures where surgeons need tooperate on a motionless heart in an almost

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Extracorporeal Blood Oxygenation Devices, Membranes for, Fig. 1 Main components and the function of theextracorporeal membrane blood oxygenation system

738 Extracorporeal Blood Oxygenation Devices, Membranes for

bloodless surgical environment. Cardiopulmo-nary bypass (CPB) is a technique in which aheart–lung machine temporarily takes over thefunction of the heart and lungs during surgery,maintaining the circulation of blood as well asthe oxygen and carbon dioxide contents of thebody. Figure 1 shows the main components andthe function of the heart–lung machine. In theoperating theater, the heart–lung machine pro-vides mechanical circulatory replacement of theheart by means of a pump and of the lungs througha blood oxygenator (BO). During CPB, the sur-geon inserts large catheters or cannulas into thevena cava and the aorta and then administers achemical solution to cause the heart to stop beat-ing (cardioplegia). Oxygen-depleted blood isdiverted from the upper chambers of the heart toa reservoir in the heart–lung machine. From there,the blood is then transferred to the BO whichinfuses the blood with oxygen and allows carbondioxide to diffuse in the opposite direction. Other

components of the bypass machine include spe-cial filters that capture air bubbles and a heatexchanger that cools the blood during surgeryand warms it at the conclusion of surgery to pre-pare for returning the body to its own cardiovas-cular circulation. The blood is cooled to maintainthe body’s metabolism at a lower rate, reducingthe oxygen consumption. At the conclusion of theoperation, the surgeon withdraws the cannulasand restores the flow of blood in the body andthe heartbeat.

The heart–lung machine is also used in inten-sive care units and cardiac catheterization labora-tories, as an extracorporeal life support (ECLS)system for maintaining blood flow and respira-tion. The diseased heart or lung(s) is replaced bythis technology, providing time for the organ toheal. The heart–lung machine can also be usedwith venoarterial extracorporeal membrane oxy-genation (ECMO), which is used primarily in thetreatment of lung disease and is considered a


E

standard therapy for the treatment of respiratoryfailure in neonatal patients (Grenvik et al. 1999;Roy et al. 2000).

In adult and pediatric patients, clinical ECMOis a treatment of last resort for individuals whowould otherwise die despite maximal therapy(Bartlett et al. 2000). Since the 1990s, this treat-ment has grown continuously in the treatment ofadult patients with respiratory dysfunction, foroxygenation and carbon dioxide removal, and asa lung-protective ventilatory strategy in order to“rest” the lungs and provide optimal conditionsfor the recovery of lung function (Ghoshet al. 2009). Despite the current limited applica-tion of ECMO in adults, a randomized controlledtrial (RCT) is recruiting patients in the UnitedKingdom to evaluate the cost-effectiveness andclinical benefit of modern ECMO technique(Peek et al. 2006). This RCT will be one of thefirst RCTs performed in adults since the bench-mark NIH trial during the 1970s (Zapolet al. 1979) and could reveal an increase in therate of survival of patients with severe, but poten-tially reversible, respiratory failure without severeoutcomes of disability, if technical and clinicaladvancements improve outcomes.

Clinical Practice, Risks, and Historyof Extracorporeal Blood OxygenationBlood is a complex tissue designed to sustain thebody by continuous circulation within a vast net-work of blood vessels coated with endothelialcells. These unique cells simultaneously maintainthe fluidity of blood and ensure integrity of thevascular system. Contact with the surgical woundand diversion of blood into the heart–lungmachine trigger a massive defense reaction thatstimulates all types of blood cells and five plasmaprotein systems to produce a myriad of vasoactivecytotoxic and cell-signaling substances into thecirculation. Platelets, neutrophils, monocytes,and endothelial cells are the major cellular actors,and complement, contact, intrinsic coagulation,extrinsic coagulation, and fibrinolytic protein sys-tems are the primary plasma participants. Whenactivated, these cells and proteins initiate complexand overlapping reactions and interactions with amultitude of target molecules to create a whole

body inflammatory response (Kirklin et al. 1983;Edmunds and Colman 2006). Although the safetyof CPB has been established with low mortalityrates, it is still associated to risks such as bloodclots and air bubbles that can cause embolism(occlusion of a blood vessel), hemolysis or dam-age to red blood cells, and immune system acti-vation or systemic inflammatory response.Studies show that presently patients undergoingcardiac surgery with CPB can still experiencesystemic inflammatory response syndrome(SIRS) (Segal et al. 1998; Onorati et al. 2010).This results in severe and irreversible effects suchas cerebral injury and cognitive impairment (Lundet al. 2005), endothelial injury (Verrier and Boyle1996), organ dysfunction and impaired hemosta-sis (Hsu 1997) also known as the process of bloodclotting, blood coagulation or thrombus forma-tion, graft occlusion, perioperative strokes andpulmonary thromboembolism (Valleyet al. 2010), and even postoperative morbidityand mortality (Hammon 2008; Vallelyet al. 2010).

In order to prevent in-circuit thrombosis,or clotting, patients undergoing CPB orECMO require the administration of systemicanticoagulation agents. Unfractionated heparin,which suppresses the action of thrombin, the keyenzyme in the thrombotic portion of the defensereaction, has been the elected anticoagulant forover half a century. Although the results are gen-erally satisfactory, there are sometimes controver-sial effects due to (i) thrombin being, in certainpatients, only partially suppressed by heparin(Janvier et al. 1996; Edmunds and Colman2006) or (ii) major hemorrhaging even after thepatients are subjected to correction of thecoagulopathy with antifibrinolytic drugs(Chalwin et al. 2008). About 30–70 % of patientsundergoing coronary artery bypass grafting underCPB require homologous blood transfusions(Scott et al. 1992). The major risks such asSIRS, activation of the blood coagulation cascade,blood cell trauma, thrombosis, clotting,hemorrhaging, etc., which patients submitted toCPB and EMCO encounter, are all due to thecontact of the blood with foreign surfaces of mate-rials with inadequate blood compatibility also


known as hemocompatibility. Among these sur-faces contacting blood in the extracorporeal bloodoxygenation system, the membranes for the gasexchange are of primordial importance due totheir extensive surface area.

The start of the historical development ofextracorporeal circulation is strongly related toCésar Julian Jean Le Gallois (1770–1814). In themonograph published in 1812, Experiences sur leprincipe de la vie, he suggested that a part of thebody might be preserved by a mechanical heartreplacement and some kind of external perfusiondevice (Le Gallois 1812). After the first idea ofexternal perfusion devices by Le Gallois, the firstattempts of oxygenating blood outside the bodywere made by the physiologist AlexanderSchmidt, in 1876, when he perfused an isolateddog kidney with oxygenated blood (Kramme2011). The original BO, invented by von Freyand Gruber (Kay 1992), in 1885 was the filmBO; it used rotating cylinders to spread blood ina continuously renewed thin film while oxygenflowed over it. Though there was direct contactbetween the blood and gas phases, the gasexchange efficiency was low, leading to devicesthat had very large priming volumes. Using thesame principle, John Gibbon, best known forinventing the heart–lung machine, developed hisfirst film BO in 1937 (Gibbon 1937) and, afteryears of tests and improvements, performed thefirst successful open-heart surgery with extracor-poreal blood oxygenation in 1953 (Gibbon 1954;Edmunds 2004). After this, several forms of sup-plying oxygen to the blood were attempted, withmore or less success, enabling the development ofmany models of oxygenators of which only a fewwere clinically employed such as the bubble BOs.In these devices, the gas exchange efficiency wasincreased by dispersing bubbles of oxygendirectly into the blood, which resulted in signifi-cantly reduced priming volumes compared to thefilm BOs (Cahn and Li 1974). On the other hand,bubbling gas through blood led to foam forma-tion, and therefore defoaming agents such assilicone compounds had to be added to theblood (Kremesec 1981). In 1955, DeWallet al. performed the first successful CPB proce-dure using a bubble BO (DeWall et al. 1966). The

DeWall–Lillehei bubble oxygenator (Lilleheiet al. 1955; DeWall et al. 1966) was a majorbreakthrough and well accepted by surgeons. Itwas a simple disposable device where oxygenbubbles were dispersed into an oxygenationchamber containing venous blood that waspumped from the patient’s body and then wasdefoamed in a defoaming compartment beforereturning to the patient. Although the benefits ofbubble versus film BOs have been often debated,both have their own limitations. For film BOs,though the blood surface area was large, the gastransfer efficiency was often compromised bychanneling of the blood flow (Niiya and Noble1985; Ho and Sirkar 1992) and a very large prim-ing volume was usually required to obtain suffi-cient gas exchange. On the other hand, bubbleBOs caused significant damage to the blood dueto the foam formation with devastating conse-quences such as denaturation of plasma proteins,gas emboli, fat emboli, fibrin emboli, andneurocognitive deficits (Lee et al. 1961, 1969;Hill et al. 1969). Despite the nonphysiologicalconditions of the blood–gas interface present inbubble oxygenators, these were used until15 years ago in around 2–3 % of open-heart sur-gery worldwide (Wiese 2010).

Membrane Blood Oxygenatorsand Principle of Membrane BloodOxygenation

Membrane blood oxygenators (MBOs) were firstdeveloped when the scientists introduced anonporous symmetric membrane between theblood and gas phases to avoid direct contactbetween the two phases in an attempt to improvethe hemocompatibility of BOs (Kolff and Balzer1955; Clowes et al. 1956; Marx et al. 1962;Dorson et al. 1968; Guidoin et al. 1978). Thisrepresented a significant breakthrough in thedevelopment of blood oxygenation as the mem-brane provides a barrier in the blood–gas interfaceand that minimizes the risk of air embolism(Iwahashi et al. 2004). Microporous MBOs are,in fact, the state-of-the-art technology for extra-corporeal blood oxygenation. Another advantage

Oxygenated blood100 mmHg O240 mmHg CO2

Oxygen depleted blood40 mmHg O2

46 mmHg CO2CO2 / O2

O2

Membrane

200 cm3 (STP) / min CO2

250 cm3 (STP) / min CO2

Extracorporeal BloodOxygenation Devices,Membranes for,Fig. 2 The principle ofmembrane bloodoxygenation


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of MBOs is in the device design and fabricationwhich, because of the defined blood channels,allows for precisely controlled blood flow charac-teristics and, therefore, possibilities for optimalefficiency in gas exchange.

As in the natural lungs, transport of gas intoblood is driven by the partial pressure differenceof the respiratory gases between the blood andthe gas separated by the membrane. Accordingto the pressure differences, gas diffuses throughthe membrane from the side of high partial pressureto the side of low partial pressure. Figure 2 sche-matically shows the principle of membrane bloodoxygenation. In order to be clinically functional foran average healthy adult weighing around 70 kg, atrest, the MBO must deliver, at standard tempera-ture and pressure conditions, approximately250 mL of oxygen per minute and remove thecorresponding carbon dioxide production of about200 mL per minute. The controlling factor in thedesign of these blood–gas exchange devices is thepermeability of the membranes toward oxygen andcarbon dioxide. The equivalent oxygen partialpressure in venous blood is about 40 mmHg andabout 104 mmHg for arterial blood. Use of air asthe supply gas and at ambient pressure gives adriving force for oxygenation of about 88 mmHg.In contrast, in venous blood, the CO2 partial pres-sure is about 45 mmHg which, assuming that theexchanging air has negligible CO2 content, consti-tutes the maximum driving force for CO2 removalfrom the blood. The initial challenge in membranetechnology was to produce reliable membraneswith high permeabilities for oxygen, and much

emphasis was placed on oxygen transfer as thecontrolling factor in the design of the blood–gasexchanger system. However, in some cases,removal of CO2 from the blood may be a limitingfactor in the design (Bungay et al. 1986; Stamatialiset al. 2008).

Development of Membrane BloodOxygenators

The very first discovery of oxygen transfer acrossan artificial membrane was made by Kolff andBerk in 1944 when they found that venous bloodwas being oxygenated while flowing through acellophane dialyzer that was in contact with oxy-gen containing dialysate (Kolff et al. 1944). Thisdiscovery stimulated the development of the useof gas permeable membranes in order to separatethe blood phase from the gas phase in the BOs.

The first plate-type membrane oxygenator wasbuilt by Clowes Jr. in 1956 (Clowes et al. 1956).Clowes’ membrane oxygenator was based on theSkeggs–Leonards (Skeggs et al. 1949; Skeggs2000) plate dialyzer with its dialysis membranereplaced with a supported ethylcellulose flat sheetmembrane. In order to have the sufficient oxygen-ating capacity for perfusion in an adult, it had tohave a multilayered system with a very largemembrane surface area of approximately 25 m2.

In 1956, Kolff developed the first coil-typemembrane oxygenator (Kolff et al. 1956b) bysubstituting the cellophane membrane in his owndeveloped artificial kidney, the Kolff rotating


drum (Kolff 1954; Kolff et al. 1956a; Kolff andWatschinger 1956), with a nonporous symmetricdense polyethylene (PE) membrane. The bloodflowed in a spiral mode, whereas the oxygenflowed parallel to the axis of the cylinder. To usethis membrane oxygenator for an adult patient, itwas necessary to assemble eight coil units with anenormous membrane surface area for effective gasexchange (Kolff et al. 1956a).

The second generation of nonporous symmet-ric membranes appeared in the 1960s with thedevelopment of silicone rubber which displayedincreased oxygen and carbon dioxide permeabil-ities. The carbon dioxide permeability of siliconeis about five times greater than the oxygen perme-ability, and this partially compensates for thesmaller available driving force for carbon dioxidetransfer (Haworth 2003). Since the diffusion coef-ficient of oxygen and carbon dioxide in air isabout four orders of magnitude higher than inblood, the gas-side mass transfer resistance wasnegligible. The major resistance to respiratory gastransfer was due to the membrane and the blood-side concentration boundary layer (Weissman andMockros 1969). This brought about a majoradvance in establishing the technical feasibilityof membrane oxygenators in the 1960s and1970s as the required membrane area could beoptimized to less than 6 m2. Kolobow took advan-tage of the development of silicone rubber and in1963 developed an oxygenator utilizing the sameconfiguration of Kolff’s coil lung but with a dif-ferent assembly. A silicone rubber envelopereinforced with nylon knit was wrapped arounda central core, and pure humidified oxygen waspassed through it under a negative pressure. Theblood flowed across the flat tubing parallel to theaxis of the cylinder. The average priming volumeof these units was of 0.1 L/m2, and they couldoxygenate venous blood at rate of approximately1 L/m2/min (Kolobow and Bowman 1963). In1965, Bramson et al. commercialized the firstdisposable nonporous symmetric silicone MBOwith an integrated heat exchanger (Bramsonet al. 1965). In 1971, Kolobow (Kolobowet al. 1971) improved his first disposable coilmembrane oxygenator by using membranes fab-ricated from silicone rubber deposited on a fabric

with an irregular structure. This membrane wasoriginally manufactured by Fuji Systems inTokyo, and its unique structure permittedenhanced secondary flow effects that improvedthe gas exchange capability. The improvedKolobow oxygenator was successfully used inthe clinical field, first as an oxygenator for CPBduring cardiac surgery, and later in respiratoryassist devices was the only oxygenator availablefor long-term application of ECMO for acuterespiratory failure patients (Kolobowet al. 1971). In 1972, J. P. Hill reported the firstsuccessful treatment of adult respiratory distresswith ECMO (Hill 1977), and after 3 years of thissuccess, the first neonatal ECMO survival casewas described by R. H. Bartlett (Bartlettet al. 1976). This was the beginning of successfulECMO application, and it has been shown to bemost effective in the treatment of newborn andpediatric patients (Bartlett et al. 1982).

The real breakthrough in MBOs came in the1980s with the development of hydrophobicmicroporous membranes. These membraneswere made from polytetrafluoroethylene (PTFE)and the membrane pore diameters ranged from0.02 to 0.1 mm. The respiratory gases pass throughthe membrane pores rather than diffuse throughthe membrane material and, consequently, offera much lower resistance to gas transfer thanthe nonporous membranes. Polypropylene(PP) microporous membranes were introducedsoon after and displaced the PTFE membranesbecause they have lower cost and better mechan-ical properties, which offered better control of theblood channel geometry (Haworth 2003).

In 1981, Y. Nose (Nosé and Malchesky 2000)started to work on the first microporous hollowfiber membrane oxygenator – the Monsanto oxy-genator. Although the gas exchange characteris-tics were reasonable, the device leaked a largeamount of plasma through the membrane walls.The first commercially available hollow fiber oxy-genators, the Capiox MBO, composed of PPmicroporous membranes was developed in 1981by Terumo Corporation (Terumo Corporation,Tokyo, Japan) and Suma (Suma et al. 1981). Inorder to avoid plasma leakage, the inner surface ofthe microporous PP hollow fibers was coated with


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a silicone layer with a thickness of approximately0.2 mm. In these two MBOs, the blood waspumped inside the bundle of hollow fibers(luminal flow), while the gases circulated on theouter side. As the venous blood entered each fiber,it was surrounded by an oxygen-rich ventilatinggas mixture which passed through the fiber wallsinto the bloodstream (Weissman and Mockros1968; Dutton et al. 1971; Suma et al. 1981;Tanishita et al. 1994). It was soon found that, forroutine short-term perfusion, these types ofMBOspresented no real advantage over bubble modelsmainly due to thrombus formation within thefibers (Calafiore et al. 1987; Bergdahl andBergdahl 1989). In the 1980s, microporousMBOs with intraluminal flow accounted for only20 % of all blood oxygenators sold in the UnitedStates (Voorhees and Brian 1996). At that time,the gas transfer performance of MBOs was not yetcomparable with that of bubble blood oxygena-tors, and, furthermore, the MBOs were complexto operate.

The disadvantages presented by the MBOs trig-gered a series of studies on mass transfer efficien-cies, pressure drops, and shear stresses throughblood flow paths in different module configura-tions. Designers of flat sheet MBO modulesfocused on incorporating passive mixing of theblood to reduce the blood-side resistance to gastransfer, and in 1984, an oxygenator was intro-duced in the market by Cobe Cardiovascular(Cobe Cardiovascular, Arvada, Colorado), with ahigh gas transfer efficiency and a membrane sur-face area of only 2.5 m2. This was achievedthrough the introduction of spacers over the flatsheet microporous membranes to induce mixingon the blood side (Elgas and Gordon 1984). Exten-sive studies of MBO modules with hollow fiberintraluminal blood flow (Alpha et al. 1986;Gassman et al. 1987) led to the design of hollowfiber modules with blood flowing on the outside ofthe hollow fibers (extraluminal flow), while oxy-gen was administered through the inside of themembranes. In 1985, the Johnson and JohnsonExtracorporeal Maxima Oxygenator (Johnson andJohnson Cardiovascular, Anaheim, California) waspresented as a highly efficient and compactMBO. It contained microporous cross-wound

hollow fibers where the blood flowed over andunder the outside of the fibers (extraluminal flow)while oxygen crossed fromwithin the hollowfibersinto the blood (Iatridis et al. 1985). In the 1980s, theKuraray Corporation (Osaka, Japan) began on thedevelopment of a new device – the MENOXoxygenator – that contained a silicone-coatedmicroporous polyolefin membrane. The reasonbehind the silicone coating is to reduce the plasmaleakage through the membrane pores and to correctany defects at the surface of the membrane. Thedevelopers claim that the membranes arenon-plasma-leaking and that the MENOX oxygen-ator could be used in ex vivo experiments for up to5 months (Nosé 2001). Another coating developedbyMedtronic Inc. (Minnesota, USA) comprised ofan alkoxysilane/alkylsilane copolymer, preferablyaminoalkylsiloxane, was used to coat microporoushollow fiber membrane blood oxygenators,increasing the resistance of the fibers to passageof blood plasma through the micropores (Plunkett2002).

Nevertheless, in both cases, the coating pro-vides an additional layer or “second skin” andintroduces an additional barrier to the gas transfer,altering therefore a severe limiting factor of theoxygen mass transfer to the blood.

The technological advances of MBOs devel-oped during the 1980s permitted the increase ofthe gas transfer efficiency and reduction ofmembrane surface area and priming volume. Asa consequence of these improvements andwith the general migration from blood flowing inthe luminal to the extraluminal space of hollowfibers, by 1992, MBOs accounted for more than98 % of all blood oxygenators sold in the UnitedStates.

State of the Art of Commercial MembraneBlood OxygenatorsThe technical and medical progress of MBOs hasbeen intrinsically associated to the developmentof membranes that can assure physiological trans-port rates of oxygen and carbon dioxide and ofmodule configurations that are compact with min-imal membrane surface area per unit volume andwith low resistance to oxygen transfer, particu-larly on the blood side.


A membrane, in a very general sense, is asemipermeable barrier that can be made of inor-ganic or organic materials. In MBOs, organicpolymers are the membrane materials of selectiondue to their physicochemical and structural versa-tility. According to their structure, membranes canbe divided in four main categories: nonporoussymmetric membranes, microporous membranes,integral asymmetric membranes, and compositeasymmetric membranes. All of them can beprocessed in the form of flat sheets or hollowfibers (Bungay et al. 1986; Mulder 1996; Baker2004). Nonporous symmetric membranes consistof a dense film where the selective transport ofgases is governed by a solution/diffusion mecha-nism. Microporous membranes have a rigid,highly voided structure with randomly distrib-uted, interconnected pores of different shapesand sizes from 0.01 to 10 mm in diameterdepending on the preparation process. Asymmet-ric membranes, first developed by Loeb andSourirajan in 1963 (Loeb 1981), consist of twolayers: a dense and nonporous symmetric active“skin” layer with thickness ranging from 0.1 to50 mm and a relatively thick porous support layerwith thickness between 50 and 500 mm. The over-all performance of the membrane is governed bythe top dense layer, while the porous sublayerprovides mechanical strength to the membrane.When the material of the top layer and poroussublayer are the same, the membrane is called anintegral asymmetric membrane and when com-posed of different materials known as asymmetriccomposite membranes.

The packaging and arrangement of the mem-branes play a fundamental role in the developmentof MBOs. Efficient packaging of large surfaceareas of membrane into small volume deviceslead to MBOs of high gas transfer rates and lowpriming volumes. Furthermore, the blood flowpath must be carefully designed. While disruptingthe blood-side mass transfer boundary layer willlead to higher gas transfer efficiencies, it alsoleads to increased shear stresses on the bloodcells. Damage to blood cells depends both on theapplied shear stress and on the time for which theshear stress is applied (Zydney 1985). Thus, whilemixing on the blood side is desirable, it is essential

to ensure that the blood cells and blood compo-nents are not damaged.

Early microporous MBOs contained up to5.5 m2 of membrane surface area (Von Segesser1987), so, in order to overcome slow gas transferrates due to the high blood-side mass transferresistance, modernMBOs have utilized secondaryflow effects to disrupt the blood-side concentra-tion boundary layer and hence reduce the mem-brane surface area and priming volume. In thecase of flat sheet MBOs, such as the Cobe CML30 (Cobe Cardiovascular, Arvada, Colorado,USA) (Voorhees and Brian 1996), the membranesurface area has been reduced to between 2.5 and3.0 m2, whereas for hollow fiber designs withextraluminal blood flow such as the Cobe Optima(Medtronic Maxima, Medtronic, Anaheim, Cali-fornia, USA) (Ulrich 1990), the membrane sur-face has been reduced to between 1.7 and 2.3 m2.

In developed countries, the rate of growth ofthe elderly population is exceeding the growthrate of the rest of the population, and this meansthat in the near future cardiac surgery will beperformed on older and less healthy patients,some of which may even need to undergo morethan one operation (Voorhees and Brian 1996).This changing patient population as well as clin-ical and economic factors will demand improvedMBOs. Designing improved and novel MBOs is acomplex procedure given the interdependence ofthe important design variables as describedbelow:

• Maximizing the gas transfer per unit primingvolume of the device is essential. In the past,the patients’ body temperature was lowered bycooling the blood during CPB, thus reducingthe oxygen requirement. In current practice,there is a trend toward normothermic coronaryperfusion which increases the oxygen require-ment (Galletti 1993). In addition, the need tominimize the transfusion of donated blood dueto possible transmission of pathogens willdrive module design toward lower primingvolumes (Voorhees and Brian 1996).

• Minimizing the membrane surface area of thedevice has clinical and economic benefits. Dur-ing CPB surgery, hematological and immune


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responses are observed as a result of contactbetween the blood and oxygenator surfaces.While these side effects are relatively minorin healthy patients, they are of concern forolder and less healthy patients. An increasedgas transfer efficiency would allow a reductionin membrane surface area and hence a reduc-tion in the average contact time between theblood and foreign surfaces. Since the mem-brane is often the most expensive componentin a MBO, minimizing the membrane surfacearea will reduce manufacturing costs.

• The blood flow path must be carefullydesigned. While disrupting the blood-sidemass transfer boundary layer will lead tohigher gas transfer efficiencies, it also leads toincreased shear stresses on the blood cells.Damage to blood cells depends both on theapplied shear stress and the time for whichthe shear stress is applied (Kolff et al. 1944).Thus, while mixing on the blood side is desir-able, it is essential to ensure that the cells arenot damaged.

In the last decade, rather than focusing on theresearch of novel blood oxygenation membranes,most researchers have been concerned with bloodflow configurations for mass transfer enhance-ment in hollow fiber MBOs. Extraluminal bloodflow may be concurrent, crosscurrent, or counter-current to the flow of gas within the fibers, and allthese flow patterns have been studied. Someresearchers demonstrated that a hollow fiber mod-ule, in which the hollow fibers are perpendicularto the blood flow, is the most efficient as it pro-motes the disruption of the momentum and con-centration boundary layer on the blood side. Thesubsequent decrease of the resistance to gas masstransfer leads therefore to the increase of oxygentransfer rates (Mockros and Leonard 1985; Yangand Cussler 1986, 1986; Rajasubramanianet al. 1997; Wickramasinghe and Han 2005).However, this blood flow arrangement compli-cates the design of oxygenators because ofthe complex blood flow path and thenon-applicability of conventional mass transfercorrelations. As a consequence, many studies ofmomentum and mass transfer in MBOs have been

carried out (Vaslef et al. 1994; Gabelman andHwang 1999; Wickramasinghe and Han 2002;Taskin et al. 2010). Nagase et al. (2005a, b) havethoroughly investigated the relationship betweenthe oxygen transfer rate and hollow fiber arrange-ment and found better mass transfer performancein parallel flow hollow fibers. This was due to thefact that in the hollow fiber module with bloodperpendicular flow, the hollow fibers in the frontlayer conceal those in the rear layer at the cross-over point, and this decreases the mass transferrates. Furthermore, the membrane surface areas ofthese parallel modules were larger than the ones inthe other modules. Other studies show that highgas transfer rates can be obtained when micropo-rous hollow fiber membranes are arranged in anorderly way in cross-laid double layer mats at anoptimal membrane angle (Catapano et al. 2001).

The MBOs used in ECMO for long-term lungsupport system applications in infants and adultsaccount for a very small part of the MBOs usedworldwide. The flat sheet coil-type modules aregenerally composed of a nonporous, dense flatsheet reinforced silicone rubber membrane enve-lope in a spiral coil around a polycarbonate spool.The blood flows between turns of the envelope ina thin film, and oxygen from the gas compartmentdiffuses through the membrane into the blood-stream. At the same time, carbon dioxide diffusesthrough the membrane into the gas compartmentand is flushed from the oxygenator by the oxygenflow. More recent oxygenators aimed for long-term ECMO usage are based on hollow fibersilicone-based membranes with extraluminalblood flow (Kawahito et al. 2002a, b; Motomuraet al. 2003).

Hollow fiber and flat sheet MBOs have almostequivalent hemocompatibility and gas exchangeperformance. However, the gross air handling isdifferent due to differences in the packing densitydefined by the total available membrane surfacearea per unit volume. Gu et al. (2000) comparedflat sheet and hollow fiber MBOs in terms ofpressure drop, shear stress, and activation of leu-kocytes or white blood cells. They found that bothconfigurations displayed similar gas transfer per-formance. However, the pressure drop along theblood flow path of the flat sheet MBOs was higher


than that for hollow fiber MBOs. Moreover, acti-vation of leukocytes in flat sheet MBOs wasgreater. De Somer et al. (1996) compared flatsheet and hollow fiber MBOs in terms of hemoly-sis, shear stress, and white blood cell and plateletcounts and found that although they have importantdifferences in pressure drop and shear stress, nostatistical differences were found in hemolysis gen-eration or other tormented blood elements. There-fore, pressure drop as a single element may not beconsidered to influence hemocompatibility.

A tremendous amount of research, asevidenced by the numerous scientific articles andpatents, has been devoted to the design of theMBOs available today. In 1999, the total BOmarket was worth about $460 million and by2001 had risen to over $500 million (Hanft2002). In 2008, an estimated 1.4 million MBOswere used worldwide for acute surgical CPB,corresponding to a consumption of approximately3 million km2 of oxygenation membranes (Wiese2009).

Presently, most of the MBOs sold worldwidecontain hydrophobic microporous membranes,and more than 95 % of them are composed ofextraluminal flowmicroporous hollow fiber mem-branes with pore sizes below 0.1 mm, outer diam-eters between 300 and 500 mm, and surface areasof approximately 2 m2. The hollow fiber mem-branes are assembled into carefully spaced cross-laid double layer woven mats which provide uni-form extraluminal flow channels for the blood(Hanft 2002). The main polymers used in theproduction of the blood oxygenation membranesare polyolefins such as polypropylene (PP), poly-ethylene (PE), and poly-4-methylpentene (PMP)and, at a smaller scale, polyvinylidene fluoride(PVDF) and silicone rubber (Wiese 2009).

Current Problems

Extracorporeal membrane blood oxygenators(MBOs) used in the medical field to substitutethe respiratory function of the lung have at theirvery core membranes that act as a blood–gasbarrier and ensure oxygen delivery and carbondioxide removal from the patient’s bloodstream.

In order to be technologically feasible, the MBOs,with minimal membrane surface area, have tocomply with two main properties: (i) be efficientgas exchangers and provide physiological oxygenand carbon dioxide transfer rates of approximately250 cm3 per minute and 200 cm3 per minute,respectively, at standard temperature and pressureconditions and (ii) be hemocompatible.

The majority of current MBOs are made ofhollow fibers of microporous hydrophobic mate-rials with pore sizes below 0.1 mm. The resistanceof these membranes toward the respiratory gasesis very low as the gas exchange between the bloodside and the gas side is performed by diffusion andconvection of oxygen and carbon dioxide via theopen pores. MBOs with these types of membraneshave high gas permeabilities and are routinelyused to provide temporary cardiopulmonarybypass (CPB) during open-heart surgery and arealso occasionally used for extracorporeal life sup-port (ECLS) to substitute short-term cardiopulmo-nary function in patients with respiratory failure.One of the drawbacks of microporous hydropho-bic membranes is plasma breakthrough whichoccurs when plasma protein layers are formed atthe membrane surface and grow through the mem-brane pores increasing the surface energy of thepore, allowing plasma to leak through from theblood side to the gas side (Montoya et al. 1992).

Another main concern that has troubled themembrane industry for the past 30 years and todate remains a serious issue is the blood compat-ibility or hemocompatibility of the oxygenationmembranes. The definition of biocompatibilityhas been updated for over 20 years and can beconsidered as being the body’s acceptance of thematerial, i.e., the ability of an implant surface tointeract with the cells and fluids of the biologicalsystem and to cause exactly the reactions whichthe analogous body tissue would bring about(Ratner 1993; Williams 2008). Presently, the bio-materials community has been unable to moreaccurately assign the term “blood compatible” toa given biomaterial in spite of 50 years of inten-sive research on the subject. There is no clearconsensus as to which materials are “blood com-patible” as blood material interactions are com-plex and involve multiple factors including


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surface chemical composition, charge, flexibility,wettability, and conditions of blood flow(Cazenave 1987). Furthermore, unlike biocom-patibility assays, there are no standardizedmethods to assess blood compatibility (Ratner2007). Labarre’s (2010) tentative definition of ablood compatible surface is “a surface able tokeep under control coagulation and inflammationprocesses at its interface with normal blood, ingiven hemodynamic conditions.” According toRatner, we still do not have truly blood compatiblesurfaces after 60 years of intense research of bloodcompatibility and after numerous approacheshave been tried (Ratner 2007). The hemo-compatibility of biomaterials is known to bedependent on various surface properties such assurface morphology, surface chemical composi-tion, surface charge, surface wettability, and sur-face topography. The overall blood-contactingsurfaces must avoid damage to the blood cellsand components but also to avoid stagnation andlocalized clotting.

To improve blood compatibility of transportsurfaces such as the ones in MBOs, researchershave focused mainly on surface modificationtechniques and producing hemocompatible sur-face coatings. Minimization of blood/materialinteractions can be achieved through chemicalmodification; however, these methods are stillfar from perfect. Surface chemical modification,besides requiring rather complex experimentalprocedures and involving high costs, still doesnot offer surfaces with long-term stability andleads to potential complications downstream(Klee and Höcker 2000). Most manufacturershave focused on the development ofanticoagulant-based coatings for the entire bypasscircuit including the oxygenation membranes.This approach led to the commercialization ofmicroporous-coated membranes and bypass-coated circuits. Various manufacturers claimthat the added layer of material provides ablood-surface interaction with improvedhemocompatibility compared to that seen forthe bulk fiber materials. For example, heparin isknown to have good blood compatibilityproperties and has been used as a coating onseveral commercialized products. Unfortunately,

heparin-based coatings are inherently expensiveand complex due to the use of sodium heparin, acomplex, costly, and fragile biologically derivedsubstance. The Carmeda Bioactive Surface(Medtronic, Inc.) uses a polymer primer coatfollowed by covalent attachment of heparin toprimary amino groups present in the primer coat(Stenach et al. 1992; Spiess et al. 1998). TheDuraflo Bonded Heparin Surface (Baxter, Inc.)uses a heparin-polymer blend-based coating thatis intended to achieve similar results (Toomasianet al. 1988; Mottaghy et al. 1989). Other heparin-based coatings intended to improve blood com-patibility are available from other medical devicemanufacturers such as Terumo (Fukasawa 1983),3M Sarns (Zacour 1988; Fried and Bell-Thomson1992), etc.

Despite these developments, in order to pre-vent in-circuit thrombosis, or clotting, whensubjected to extracorporeal blood oxygenationusing any of the MBOs available on the market,patients still require the administration of antico-agulant drugs that often lead to major hemorrhag-ing and blood transfusions.

Another approach to improve the hemo-compatibility of membranes destined for MBOsfocuses on the development of novel blood oxy-genation membranes made from materialsdifferent to the polyolefins mentioned before.Polyurethanes (PUs) first found a niche in bio-medical applications mainly because of theirunique mechanical properties, particularly fatigueresistance, tensile strength, and abrasion (Szycherand Poirier 1983; Szycher et al. 1983; Graig 1983;Gogolewski 1989; Hepburn 1992; Furukawa1997), but also because of their good bio- andhemocompatibility properties (Boretos 1980;Wang and Cooper 1983; Yoon and Ratner 1988;Gogolewski 1989). PU membranes are a goodcandidate for blood oxygenation devices as thetype, length, ratio, and crystallinity of the differentmonomers as well as the method of membranepreparation determine the bulk properties respon-sible for the final mass transfer properties (Yilgörand Yilgör 1999; Hoshi et al. 2000; Jonquièreset al. 2002) and the surface properties that affectbio- and hemocompatibility (Yoon and Ratner1988; Gogolewski 1989).


Blood Compatibility and Membrane/BloodInterfacesIn 2007, an estimated 250 million polymericdevices contacting the bloodstream were used inhumans. All of these blood-contacting devicesexhibited thrombosis problems, and all cardiovas-cular device procedures, both short term and longterm, required significant anticoagulation, at highcost and with considerable risk to the patient(Geckeler et al. 1997). The number of blood-contacting medical devices used worldwide isincreasing every day, but their hemocompatibilityproperties still remain far from ideal.

To this date, there is no consensus definition ofblood compatibility. In 2007, Buddy Ratnerlabeled as a “catastrophe” the fact that the scien-tific community has still not been able to accu-rately assign the term “blood compatible” to abiomaterial in spite of 50 years of intensiveresearch on the subject (Geckeler et al. 1997;Ratner 2007). While the definition of biocompat-ibility is generally accepted as the ability of amaterial to perform with an appropriate hostresponse in a specific application (Williams1987), blood compatibility or hemocompatibilityhas received different definitions by variousauthors. From a clinical point of view, a biomate-rial can be considered as blood compatible whenits interaction with blood does not provoke eitherany damage of blood cells or any change in thestructure of plasma proteins. Only in this case canit be concluded that this material fulfills the mainrequests of biocompatibility (Gurland 1987).A tentative definition, by Labarre (2010), of ablood compatible surface is “a surface able tokeep under control coagulation and inflammationprocesses at its interface with normal blood, ingiven hemodynamic conditions.”

The contact between the blood and the surfaceof a biomaterial often leads to different degrees ofclot formation, as a consequence of thenonspecific protein adsorption and adhesion andactivation of blood cells (Horbett 1986; Courtneyet al. 1994). Within the first few seconds afterblood contacts, material surface plasma proteinsare deposited. The extent of protein adsorptiondepends on the properties of the foreign surface,namely, the wettability, charge, roughness,

chemical composition, etc. This protein layer con-trols further reactions of the other blood compo-nents and cell system. Additionally, the adsorbedproteins are subject to conformational changes aswell as exchange processes with other proteins(Ziats et al. 1988). The competitive adsorptionbehavior of proteins at the material surface deter-mines the pathway and the extent of intrinsiccoagulation and adhesion of platelets. Plateletsplay a fundamental role in hemostasis. Followingthe protein adhesion to the surface of the bioma-terial, coagulation is initiated by platelets adher-ing to specific binding sites exposed by the plasmaproteins. The activated platelets swell, grow spikyextensions known as pseudopodia, and release thecontents of their secretory granules, which containa variety of substances which include ADP, sero-tonin, and thromboxane A2, which stimulate fur-ther platelet adhesion and activation and enhancethe coagulation process. A large variety of clottingfactors take part in a series of chemical reactionsthat eventually create a mesh of fibrin fibers. Eachof the clotting proteins has a very specific func-tion, and the three main ones are prothrombin,thrombin, and fibrinogen. Thrombin promotesthe conversion of fibrinogen into long insolublefibers or threads of another protein – fibrin. Fibrinthreads wind around the platelets forming aninterlocking network of fibers and a frameworkfor the clot. This net of fibers traps and helps holdplatelets, blood cells, and other molecules tight tothe material surface, forming a blood clot alsoreferred to as a thrombus.

It has long been established, in the field ofbiomaterials, that bio- and hemocompatibility ismainly governed by the surface properties of thematerial that contacts the blood (Cooper 1982;Lelah and Cooper 1986; Schamberger andGardella 1994). Predictions about the interactionsbetween the biomaterial surface and the adsorbedproteins can only be formulated by having anexact knowledge of the structure of the biomate-rial’s surface and the conformation of theadsorbed proteins. These interactions are deter-mined both by the hydrophobic/hydrophilic,charged/uncharged, and polar/nonpolar parts ofthe proteins and the properties of the polymersurface such as chemical composition,


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morphology, charge, wettability, and topography(Dawids and Bantjes 1986; Williams and Wil-liams 1989; Stamm 2008).

Hemocompatibility properties of surfacematerials can be evaluated following theISO10993-4 (Biological evaluation of medicaldevices, Part 4: Selection of tests for interactionswith blood. Geneva, Switzerland: InternationalOrganization for Standardization; 2002) guide-lines in terms of protein adsorption, hemolysis,platelet adhesion and platelet activation, wholeblood clotting time, and clinical coagulationtimes.

Hemocompatibility properties of surface mate-rials can be evaluated in vitro according to the ISO10993-4:2002 standard (ISO standard) guidelinesin terms of hemolysis, thrombosis, platelet adhe-sion and activation, protein adsorption, and clini-cal coagulation times. The three categories ofeffects on blood most commonly evaluated arehemolysis, thrombosis, platelet adhesion, andactivation (Imai and Nose 1972; Goodmanet al. 1989; Allmér et al. 1990; Haycox and Ratner1993; Tze-Man et al. 1993; Bahulekar et al. 1999;ASTM International 2000).

Damage to the membrane of red cells, as aresult of blood exposure to foreign materials, canbe evaluated by the hemolysis test. Hemolysis canbe assessed by quantification of released hemo-globin (Hb) after contact between blood and sam-ple, according to the ASTM F 756-00 standard.The Hb concentration can be determined by col-orimetry at 540 nm, with a UV/VIS spectropho-tometer. In the cyanmethemoglobin method(Moore et al. 1981; Van Assendelft et al. 1996;Zwart et al. 1996), blood samples with a plasmaHb concentration below 2 mg/mL are diluted inPBS in order to obtain blood with a total Hbconcentration of 10 1 mg/mL. The contactbetween diluted blood and membranes with anarea of at least 7 cm2 is maintained for at least4 h of static incubation at 37 �C, and after whichthe blood is centrifuged, the supernatants are ana-lyzed for their Hb content. Two controls are com-monly used: distilled water (positive control) andPBS solution (negative control). The hemolysisdegree is expressed through the hemolytic index(HI), calculated as follows:

HI ¼ Hb½ released � Hb½ negative control

Hb½ positive control � Hb½ negative control

� 100

(1)

where [Hb]released is the concentration of Hb in thesupernatant solution and [Hb]negative control and[Hb]positive control are the concentrations of Hb inthe negative and positive controls, respectively.According to ASTM F-756, materials can belabeled nonhemolytic when the HI is between0 and 2, slightly hemolytic when HI is between2 and 5, and hemolytic when HI >5.

Thrombosis can be evaluated through an assaybased on the determination of the thrombus massformed on the surface of the membranes aftercontact with static, recalcified blood, based on amodification of the gravimetric assay proposed byImai and Nosé (1972) and Allmér et al. (1990).Samples of each membrane are cut into pieceswith a surface area of 4 cm2, and 250 mL ofblood are carefully placed on top. The clottingprocess is then started by addition of 25 mL of0.10 M CaCl2 to the blood drop on the sample.The clotting process is stopped at different timeperiods by addition of distilled water. The throm-bus formed at the surface of the membrane is fixedwith formaldehyde and then washed with water.The thrombus mass is determined after drying thethrombus at 37 �C until a constant weight wasreached. The results are expressed as a percentageof the thrombus mass formed on glass (positivecontrol). A pre-weighted filter paper disk thatfollowed the above procedure in parallel, but inthe absence of a membrane sample and of blood,was used as a blank. From the thrombus massformed on each sample, a thrombosis degree iscalculated as a percentage of the thrombus massformed on the positive control after subtractingthe blank from each thrombus mass. A thrombosisdegree of 100 % is assigned to the positivecontrol;

Thrombosis %¼ mmembrane�mnegative control

mpositive control�mnegative control

� 100

The interaction between platelets and the surfaceof the membranes can be evaluated by a techniqueoften used by researchers based on the analysis of

Extracorporeal Blood Oxygenation Devices, Mem-branes for, Fig. 3 Diagram of the platelet spreadingprocess divided into five shape categories for analysisand correlation between platelet categories and planimet-rically determined platelet spread area. From left to right,the stages of spreading are defined as follows: stage I –round or discoid: no pseudopodia present; stage

II – dendritic or early pseudopodial: one or more pseudo-podia with no evident flattening; stage III – spread den-dritic or intermediate pseudopodial: one or morepseudopodia flattened, hyaloplasm not spread betweenpseudopodia; stage IV – spreading: hyaloplasm spreadbetween pseudopodia; and stage V – fully spread: hyalo-plasm extensively spread, no distinct pseudopodia


scanning electron microscopy (SEM) images(Goodman et al. 1989; Haycox and Ratner 1993;Tze-Man et al. 1993; Bahulekar et al. 1999).Platelet-rich plasma (PRP) is prepared by centri-fugation of the pooled blood and is incubated withthe membrane samples for 30 min at 37 �C. Afterincubation, the samples are fixed with glutaralde-hyde post fixed with osmium tetroxide (OsO4).After fixation the samples are dehydrated througha graded ethanol series to 100 % and dried bycritical point drying. The dry samples are thenobserved by SEM. Quantitative and morphologyanalysis of platelets adhered to the surface of themembranes is carried out by imaging software.

In order to ensure the presence and activity of theplatelets, glass is generally used as positive controlfor in vitro platelet response to foreign materials asit promotes enhanced adhesion and activation ofplatelets (Park et al. 1991; Amiji 1998).

The indicators of platelet adhesion used areplatelet deposition (PD) and platelet coverage(PC), which are the number of adhered platelets/10,000 mm2 of membrane surface area and the

percentage of the membrane surface area coveredby platelets, respectively. The platelets are eitherpassively attached and remain discoid and singu-lar or may be activated subsequently. In the lattercase, they progressively release their granule con-tents, thereby attracting further platelets, showinga typical shape change with flattening, formationof pseudopodia, and increasing area of contact.Scanning electron microscopic images of adheredplatelets with magnifications of �1,000 or�2,000 magnification have been used by severalauthors for the quantitative analysis of platelet–material interaction. The extent of platelet spread-ing is examined by categorizing platelet shapesinto five morphological forms describing increas-ing activation stages (I–V). These are discoid orround (stage I), dendritic or early pseudopodial(stage II), spread dendritic or intermediate pseu-dopodial (stage III), spreading or late pseudopo-dial (stage IV), and fully spread (stage V). Thespreading stages and respective platelet areas areshown in Fig. 3 and defined in the legend (Allenet al. 1979; Goodman et al. 1984, 1989).


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Extraction Index

Giuseppe BarbieriInstitute on Membrane Technology, NationalResearch Council of Italy, ITM-CNR, Rende,Italy

The extraction index is a measure of membraneoperation performance of extracting a desired spe-cies from the feed/retentate side in order to have itas permeate. It can be defined/evaluated for anyinvolved species. It gives an indication about thereal advantages offered by using a membrane unitinstead of conventional ones. The higher the var-iable value, the higher the performance of themembrane unit. It is defined as the ratio (Eq. 1)of the permeate flow rate of the species permeatedthrough the membrane to that totally fed to themembrane unit. Equation 1 shows different waysfor evaluating the extraction index:

Extraction Indexi ¼ Permeate flow rateiFeed flow ratei

¼ Permeate flow rate

Feed flow rate

Permeate molar fractioniFeed molar fractioni


Feed flow rate

Permeate Partial PressureiFeed Partial Pressurei


Feed flow rate

Permeate concentrantioniFeed concentrantioni


Feed flow rate

Permeate mass fractioniFeed mass fractioni

(1)

http://onlinelibrary.wiley.com/

http://onlinelibrary.wiley.com/

756 Extremophiles

The extraction index assumes a specific and more be fed in it but also produced by reaction. In this
interesting form when a chemical reaction takesplace inside the membrane unit such as in themembrane reactors since a desired chemical can
case, Eq. 1 has to take into account also the pro-duction term:

Extraction Indexi ¼ Permeate flow rateiFeed flow ratei þ Production by Reaction for i� th

(2)

If the water-gas shift reaction is used as an example,the extraction index can be defined as the ratio ofthe H2 permeated through the membrane to that

totally available in the feed stream both as H2

molecules and also obtainable from other chemicals(e.g., CO) by reaction, to themembrane unit (Eq. 3):

H2 Extraction Index ¼ Flow ratePermeateH2

Flow rateAvailable in the feedH2

in the MR¼ Flow ratePermeateH2

Flow rateFeedH2þ a � Flow rateFeedCO

a ¼ H2O=CO feed molar ratio per H2O=CO feed molar ratio < 1

1 per H2O=CO feed molar ratio � 1

�(3)

The extraction index (Barbieri et al. 2008) takes intoaccount the hydrogen fed as H2 molecules and thatis contained in the feed stream in other chemicals(e.g., H2O). The term (a flow rate CO feed) of theEq. 3 considers the maximum H2 extractable fromthe chemicals (other than hydrogen) present in thesystem. The coefficient (a) takes into account thedefecting reactant (CO or H2O) by means of thefeed molar ratio H2O/CO. It (a) is equal to the feedmolar ratio if the latter is lower than 1 (CO in defectwith respect to H2O). It (a) will be equal to 1 whenthe CO exceeds the H2O. As defined, the extractionindex is determined by the membrane properties,feedmolar ratio, and CO conversion achieved in themembrane reactor, at set operating conditions.

References

Barbieri G, Brunetti A, Tricoli G, Drioli E (2008) Aninnovative configuration of a Pd-based membrane reac-tor for the production of pure hydrogen. Experimentalanalysis of water gas shift. J Power Sources182(1):160–167. doi:10.1016/j.jpowsour.2008.03.086

Extremophiles

Chiara Schiraldi and Mario De RosaDepartment of Experimental Medicine, Section ofBiotechnology, Medical Histology and MolecularBiology, Second University of Naples, Napoli,Italy

The discovery of life in demanding environmentscontinues to challenge conventional concepts ofthe growth-limiting conditions of many cellularorganisms. Extremophiles may have diverse fea-tures; they may live at temperatures higher than60 �C (thermophiles) or prefer colder sites, beingable to grow between 5 �C and 20 �C; they mayneed pH <4 or >9 (acidophiles or alkaliphiles,respectively); or they can survive to high-salinityenvironments such as salty lakes (halophiles).They have been isolated in challenging biotopesfrom terrestrial solfataric fields to marine volcanicareas (Rothschild and Mancinelli 2001). The

Extremophiles, Fig. 1 Electron micrograph ofSulfolobus solfataricus MT4

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phylogenetic assessment of the isolated specieswidens between Archaea, Bacteria, and Eukarya.There has been a steady increase in the isolation ofthese microorganisms documenting the enormousscientific effort in the last 30 years. Althoughmajor advances have been made in the lastdecade, our knowledge of the physiology, metab-olism, enzymology, and genetics of this fascinat-ing group of organisms is still limited. Howeverthere is little doubt that extremophiles will supplynovel catalysts and will be a source of biomole-cules with unique and biotechnologically relevantproperties. It has been argued that membranes ofextremophiles contain surfactants bearing uniquestability that can be used in pharmaceuticaland cosmeceutical formulations. Amylases,pullulanases, and glycosidases from hyperther-mophiles were studied and proved efficient instarch biotransformation obtaining processthroughput enhancement (Burg 2003).Trehalose-forming enzymes were found inSulfolobus shibatae and S. solfataricus isolatedin diverse solfatara fields worldwide (Fig. 1).Also lipases and esterases from thermophilicmicroorganisms and Archaea proved unique fea-tures (Schiraldi et al. 2002). Compatible solutes,of key importance for halophile survival, wereisolated from Halomonas and Marinococcus spe-cies and thoroughfully characterized proving theiroutstanding DNA/enzyme stabilization capacity;their commercialization was pursued by a

specifically founded company (bitop).Alkaliphilic enzymes were found as natural deter-gents, thus suggesting specific industrial interest.Psychrophilic enzymes may also be of applicativeinterest when biotransformations need to be car-ried out at low temperature. Other innovativeproducts from extremophiles are cyclodextrinsand polyunsaturated fatty acids, for whichindustrial applications are foreseen. Most ofthese kind of microorganisms isolated to dateshow specific needs to increase cell density.Recent alternatives to technological applicationsmost commonly employed when the productionof extremocompounds is approached at a bioreac-tor scale describe batch, fed batch, and continuousor in situ product removal fermentations inspecifically developed bioprocesses. However,when large-scale production is needed forcommercialization of these novel compounds,there is the need to bring together genetic andbioreaction engineering with separation tech-niques. The former prompted the developmentof specific plasmids and vectors to obtain asound expression of extremophilic gene sequenceand thus enzymes into mesophilic hosts; theserecombinant strains are easier to cultivate, andthe whole strategy is in fact recommendedto solve the typical problems faced inextremophile-based bioprocesses and willserve to open up new opportunities for the devel-opment of unexplored fields such as renewableenergies.

References

Burg BVD (2003) Extremophiles as a source for novelenzymes. Curr Opin Microbiol 6:213–218

Kristjansson JK, Hreggvidsson GO (1995) Ecology andhabitats of extremophiles. World J MicrobiolBiotechnol 11:17–25

Rothschild LJ, Mancinelli RL (2001) Life in extreme envi-ronments. Nature 409:1092–1101

Schiraldi C, Giuliano M, De Rosa M (2002) Perspectiveson biotechnological applications of archaea. Archaea1(2):75–86

Sellek GA, Chaudhuri JB (1999) Biocatalysis in organicmedia using enzymes from extremophiles. EnzymeMicrob Technol 25:471–482

758 Extremozymes

Extremozymes

Chiara Schiraldi and Mario De RosaDepartment of Experimental Medicine, Section ofBiotechnology, Medical Histology and MolecularBiology, Second University of Naples, Napoli,Italy

The biocatalysts produced by extremophilicmicroorganisms, so-called extremozymes, areproteins with outstanding stability to temperature,pH, and organic solvents, thus becoming excellentcandidates to improve industrial biotransforma-tions (Schiraldi and De Rosa 2002). Polymer-degrading enzymes from hyperthermophiles,psychrophiles, and acidophiles may play animportant role in food, detergent, and pulp andpaper industry (e.g., amylases, pullulanases,xylanases, proteases). Extremozymes also includecellulases, proteases, pectinases, keratinases,lipases, esterases, catalases, peroxidases, andphytases. Owing to the unusual properties ofthese classes of enzymes, they are expected tofill the gap between biological and chemicalindustrial processes (Taylor et al. 2011). Despitethis, actually few are the current processes basedon these biocatalysts, the most known of which isthe polymerase chain reaction (PCR) technologythat in fact is based on DNA-modifying enzymesTaq polymerase isolated from Thermus aquaticus.The use of this enzyme allowed the automation ofPCR with a great advantage for research labora-tories and industries. Beside this, few biocatalysts,used in diagnostics and starch liquefaction, arecommercially produced by several biotechnologycompanies. Recently enzymes of interest in bio-remediation have also been isolated fromextremophiles: a thermoalkaliphilic catalase,which initiates the breakdown of hydrogen perox-ide into oxygen and water, was isolated fromThermus brockianus, found in YellowstoneNational Park, and operates over between 30 �Cand 94 �C (pH range 6–10). Its outstanding

stability and wide operational range suggest appli-cation in hydrogen peroxide removal in pulp andpaper bleaching, textile bleaching, food pasteuri-zation, and surface decontamination of food pack-aging (Thompson et al. 2003). Enzymes isolatedfrom psychrophiles, such as lipases, proteases,and cellulases, have been used as additives forthe preparation of detergents working at low tem-peratures or in frozen food preparations. Further-more, thermophilic enzymes have been used forthe construction of optical nanosensors, stable andnonconsuming analytes. These innovative devicesare based on the ability of thermophilic enzymesto bind the substrate at room temperature, withouttransforming it (de Champdoré et al. 2007). Thebinding of substrate to thermophilic enzyme ismonitored as fluorescence variations of theenzyme. Many of these isolated enzymes havebeen cloned and expressed in mesophilic hoststo overcome the issues of extremophilic microor-ganisms cultivations, such as the unconventionalfermentation parameters, special constructionmaterials need, the low growth rates that are typ-ical of most of these species, and the low biomassyield (despite the good enhancement proved inmembrane bioreactors) (Schiraldi et al. 2001).The recombinant enzymes (Cimini et al. 2008),easily produced at high yield, may be commer-cialized, and furthermore, modern techniques likemutagenesis and gene shuffling will lead toin vitro tailored enzymes that are highly specificfor countless industrial applications.

References

Cimini D, De Rosa M, Panariello A, Morelli V,Schiraldi C (2008) Production of a thermophilicmaltooligosyl-trehalose synthase in lactococcus lactis.J Ind Microbiol Biotechnol 35(10):1079–1083

de Champdoré M, Staiano M, Rossi M, D’Auria S (2007)Proteins from extremophiles as stable tools foradvanced biotechnological applications of high socialinterest. J R Soc Interface 4(13):183–191

Schiraldi C, De Rosa M (2002) The production ofbiocatalysts and biomolecules from extremophiles.Trends Biotechnol 20(12):515–521

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Schiraldi C, Acone M, Giuliano M, Cartenì M, De Rosa M(2001) Innovative fermentation strategies for the pro-duction of extremophilic enzymes. Extremophiles5(3):193–198

Taylor MP, van Zyl L, Tuffin IM, Leak DJ, Cowan DA(2011) Genetic tool development underpins recent

advances in thermophilic whole-cell biocatalysts.Microb Biotechnol 4(4):438–448

Thompson VS, Schaller KD, Apel WA (2003) Purificationand characterization of a novel thermo-alkali-stablecatalase from Thermus brockianus. Biotechnol Prog19:1292–1299

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effective diffusivity - springer · miller gq, stöcker j (1989) ... ies, such as ions dissolved in...

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